Rythmik 15 sealed sub build - two of them

[dan77]

The beginning, September 08
I begin my journey into DIY speaker building, although this is not my first time. Either way, research is the key word here. I begin researching all the different types of subwoofers out there. Even though I knew I wanted to build my own I carefully read about ready-made subs as well. I followed up on all suggestions by friends and any suggestions on the web. Especially those discussed at the Shack. After much reading about who was raving about what I decided to go with a servo sub. Further I decided to go with 15's, just in case I needed the extra SPL . They will be a 3 cubic foot sealed enclosure. The DS1500s model. The external dimensions are 18 inch wide, 22.5 inch deep and 26.5 inch high. I will be using three quarter inch MDF. For the price, Brian Ding has the best. Brian runs Rythmik Audio. Ikka described the sub as ‘dry’. To me that means you can feel the bump of each sine wave. No mush at all. Also, these subs are touted as being some of the most accurate out there. I don’t like just ‘more bass’ I want clean bass. For that reason I decide that I am going enclosed. The last subs I built (about 14 years ago) were ported and I always felt there was room for improvement.
Here are some pics of my original subs. I made them look like Greek columns as I liked that look, and tubes get rid of alot of bracing issues.

Closeup of the paint I used. A can spray that simulates granite or something like that


4 months later (Early March of 09)
I have spent months off and on reading about all the different types of subwoofers out there. The options are daunting. I am still gleaning that Rythmik Audio stands out for what it is competing against. I don’t have 3 grand to spend on a single speaker, but from what I read, Rythmik Audio subs can compete with far more expensive subs. that’s all I need to know! I have the go ahead from my wife to spend the money and make my purchase. Brian was very nice to work with on the ordering.
One week later
The subs arrive. Large boxes but they fit into my Golf readily. One was a little crushed on the corner so I opened each box and inspected everything before leaving the post office. It was all packed quite well and all the items looked good. When you purchase two and pay by check, you get discounts. Although you have to wait for your check to clear, the discount (for me) was worth it. Both subs were a total of $1100.

I think the speaker looks awesome

The amp is exceptionally heavy

Day 1
Purchase MDF from home depot. Set it aside in the garage. three quarter MDF is very heavy.
Day 2
I spend the evening drawing out where on the MDF boards I will make my panel cuts. I am building two subwoofers, so everything is double. The panels are horribly heavy and I am doing this project by myself. I work it out so that I can cut each of the ¾ inch MDF boards into 4 sections each. Then I can cut each ¼ section into two panes each. Each ¼ section is very easy to lift and feed into my radial saw. This is very important as I predict it will help me to get slightly more accurate cuts by not fighting a large section of MDF.
Here is my scritching, my modest attempt to keep things organized

Day 3
Today is my first day of actual building. I pull out onto the lawn one of the ¾ inch boards and begin cutting it into fourths, per my drawings. Each fourth board is quite easy to tote around and feed into the machine.


For today, I am able to cut all the major panels.

I also get the main baffle piece cut.

This is the Radial saw I used

I am then able to assemble the pieces together and complete both boxes. I use one inch and a quarter deck screws to hold them together. I realize that glue alone will work, but I don’t own a bunch of clamps and waiting for the glue to dry takes quite a while. They are now glued and screwed and slowly drying. I did not cut the circles for subs themselves yet. All this took me the entire day, about 10 hours.
Here is the test fitting of sub number one

Just before gluing it shut, my cat performs a quick quality control check

The screws cause the glue to eak out

A slightly better shot

Here are both boxes, only the bottom panels are missing

Here I am figuring out where the amp cut out goes

Cutting it out

Test fit with the amp


Day 4
Today turns out to be baffle day. The plans from Rythmik are nice and detailed, but cutting each piece and hand fitting them all into each box ends up taking the entire day. Today I spent roughly 11 hours. First I measure each cut, then I cut the holes in each baffle piece, then I route out each hole. On this day my friend Quentin stopped by and helped me with the routing



This was by far the most detailed workday. By the end of the day I was a little burn’t out from all the cutting and sanding. After today, it will be mostly assembling and minor cutting. Although I still have to figure out how to cut the 14 and 15 inch speaker holes. Remember, I have no wood workshop as I see in so many posts here, but I have a few key tools and a bit of creativity.
Here the main baffles are cut

These are the side baffles as outlined in the Rythmik pdf

These are the other side baffles that make up the entire baffle system

Before cutting all the circles in the baffles I run a test fit

Various pictures of cutting the circles in the baffle pieces




Day 5
From this point on I will only have a few hours at a time to work on my speakers. So on this evening, I once again test fit the inner baffle panels and begin gluing them in. Along the way I add the sound deadening polyfill. Today I only get the top third of the baffles in along with the polyfill.

Here are all the pieces together in one shot. My inspectors don't miss a beat.

Here goes the pollyfill and the primary baffle

An extra layer of polyfill for the top of each sub. I hope a little glue will hold it in place

I fold it in half to slide it in

Day 6
With just a few hours available, I work on the next level of baffles. Adding more polyfill as I go. I glue the polyfill to the box walls with silicone caulking.
Here are the two boxes finished with baffles and poyfill up to the two thirds level

As I take them to the garage for sanding and routing, I stop to weigh them. They come in at 70 pounds each

Now I am done with routing the edges and sanding the sides

Tried to get a little fancy towards the bottom

Day 7
Tonight I tackle the issue of cutting large circles in MDF. I go to home depot but they have never heard of such a guide for a router. Of course, after building my own I find on the Shack site several recommendations and they all look good. But, at that point I had already made the cuts. This was the first time in the project that I had to re-cut a board. I cut the circle too large. However given the crudeness of what I built I guess something like that would happen. I started over and got the next 4 circle cuts dead on. That took the evening.
Here is my home made router circle cutter

Day 8
Tonight I work on the feet. I found some furniture legs at home depot that if cut down I thought would look good. I had ordered some black chrome spikes that will match up well with their dimensions. Each foot is drilled for the threaded rod from the spike then drilled for the hurricane nut looking thing that the spike screws into and then routed so the hurricane nut will be recessed enough for a flush fit of the spike. Finally I cut the tip of the furniture leg off so that I will have 3 inches of clearance including the wood and the spike. The plans call for 2.5 inches, but I decided to add half and inch. This was the only place that I deviated from the plans from Rythmik.
Here I screw the furniture foot to a 2 by 8 piece of lumber and determine how high to cut it down to


Here is the result, eight feet. Here I have already placed the thread anchors into the wood, and glued them there

Closeup of the anchor

Here I test fit the feet along with the black chrome spikes to see how it will look

Day 9
Tonight I clamp the inner and outer speaker boards together (with glue and screws) then attach that assembly to the box and glue/screw that down. Next I attach the feet I made last night. I think maybe they are too small in width for the size of the box, but at this point I simply move on. With this concern in mind I make the feet attach with no glue, only 2.5 inch counter sunk deck screws so that later I can switch them out if I want to.
This is just before I clamp the inner baffle to the outer baffle. Lots of glue as they need to be as one

Here is the glue that 'eaked' out

Test fitting the speaker!

I think the speaker fits a little to tight. Less than one sixteenth from edge of speaker to outer baffle. Oh well!


Day 10
The boxes are all sanded routed and smoothed out. Let the painting begin! I use ‘Sandwash’, a paint found at home depot. It looks like mud and the final look resembles something like 60 grit sandpaper. I like that kind of finish, so I go for it. I get ‘Obsidian Black’.

Day 11
Another coat of paint in the morning. They are starting to look really good, the color and texture is amazing. Being a thick paint, all blemishes are hidden and they have a texture that is pleasing to the eye. After the paint dries, I swash on a clear coat on. Its says to use a ‘roller of high quality’. As it goes on it seems rather visible, but I figure it will clear up. 4 hours later and the finish looks horrible. Almost every roll seems to show. It has milky white stripes everywhere. I stare at the other sub and think that maybe I should just not give it a clear coat. The problem is that any contact to the paint leaves visible marks. The first time I move it will certainly ruin much of the paint job. So I decide to apply a very thin layer of the top coat and that is when it hits me! I should be using a brush. This was the magic bullet, the finish went on thick and absolutely invisible. The second sub looked great. Now I have to go back and re-paint the other sub as it looks ridiculous. I will let them sit overnight so that everything can dry, glue, paint, etc. I have read that caulking/glue etc, can possibly eat at the speaker surround. So I place a heated fan blower pointed at both of them and let them cure overnight for good measure.
I blow a heater into them for a half day each to dry everything out:

Just had to see one against our 10 inch ported sub that powers our living room 5.1 setup. This was a great lesson on the mechanics of speakers. The little one would go just about as deep and was just as resonant, at low volumes. The only thing the Rythmik had to offer was more spl. I say that, however I was only listening to the radio, I had no REW for backup. I am sure it wont go down to 20 hz. But for radio listening at low levels it was very competitive. For casual evening tv watching it works quite well.
I also have to say that the Rythmik was picking up on some line noise that was very noticable. The little one was competely silent. Once I moved the Rythmik into the basement there was no line hum. Not sure why upstairs has that.

Day 12
I give the re-painted sub a clear coat (matt finish) using the paintbrush, not the roller. It looks just as good as the other sub now. The only difference is that this sub now has 6 coats on it. Oh well!
Finally, late in the day I am able to attach the black chrome spikes and take a look.

I have no idea how you are supposed to attach a 15 inch grill to a 15 inch hole, as you can see I pulled a true DIY here. Had I known that in the end these speakers would be staring straight at me, and not down, there is no question I would have come up with a more cosmetic solution. But after placing these in my home theater, REW told me the best placement was pointing at me. I tried almost two dozen locations.

The finished product! I set each one next to my olds subs for a visual comparison. The new boxes are coming in around 100 pounds and are fairly difficult to move around. But they do look like they mean business!
Sub one:

The subs are not quite black, but they are not as grey as they appear in these photos.
You can see the Crowson under the couch foot to the left of the Rythmik. Don't get me started on how much these add to the movie experiance. Tactile is something that has to be felt to be believed (pun intended!)
sub two:

Day 13 (one week later)
I finally have some time to run a dvd. I was running my old subs with a 15 year old Kenwood and pre-amp. I simply disconnected the 12 gauge wire and hooked them to the Rythmik’s. I was not pleased. It was way to easy to over power them and the bass was no better than what I had built 15 years ago. So I decided to go for a ‘proper’ speaker hookup, meaning attach them via the RCA inputs. This meant also re-wiring the fronts through the Kenwood amp to power them as I needed to run them through an EQ to help prop up their bass. Now however, the receiver was not controlling the balance of the surround set up so getting all the speakers to have appropriate sound levels took some time. I needed to run the front mains through a mixer so I could bring up the 80-150 Hz range. The EQ is some ancient garage sale special, 12 bands per channel. The front mains are 12 inch ported 1970’s technics. The are about 3 feet tall and very heavy. I don’t know if they are good or not, but the manufacturer was confident enough to put a plate stamped on the top showing the frequency response from 20-20. Looks pretty official anyway, also the tweeters look pretty tough. Either way, when I ran REW a few months ago it showed that without equalizer intervention, the lower end was a little weak. My center channel is an infinity center channel. The surrounds are infinity bookshelf speakers that I use as surrounds. My projector is a DLP Sharp 12000-U. DVD player is a Wal-Mart special, and we know Wally only sells the best, right?

Once I had all the re-wiring completed I sat down with “War of the Worlds”. I must say that after some crossover and channel level tweaking, I was very happy. Eventually I got to “Jumanji”. Overall not such a good movie for sound (for me anyway), but when the lion appears and growls for the very first time I was shocked. First, it is a very deep sound, and very clear. A good test of a sub-woofer. I must say that no matter how high I turned up my old subs I never got the furnace venting to rattle. But first pass with these subs and the rattling was so bad I had to take a timeout and fix as much of that as I could and then listen to the scene over again! (and again!) The vibrations in the metal competed far too much with the sound effects. J
I must say that re-balancing the system with this whole new setup took many evenings of tweaking and experimenting.
These speakers have taken the whole listening experience to a whole new level. I am very pleased with the outcome. Also, I have to mention that I bought some Crowson Technology Transducers last winter. My wife must have had a fever when she agreed to that purchase, but she did and it made me very happy! I say that as they run $1500 bucks for a pair. I read for months about that purchase as well. Many reviews stated that with a good enough sub you don’t really need them. Now, I know I did not by some three grand a piece subs, so maybe I speak out of turn here, but nothing can replace TACTILE sensations. Yes, my new subs can make the fabric and very frame of the couch move in motion with the frequency, but having the ground shake from below you really makes the opening scenes in WOTW come to life. Until I can afford three thousand plus subs, I will probly always have that opinion.

Here is my home theater. It is 16 wide 21 deep and 9 high. It is completely unfinished, however I bought $70 worth of black material from Hobby Lobby and hung it on all the walls. The material behind the screen (104 diagonal if I remember correctly) is a photography muslim. The projector is just sitting on a bookshelf and the carpet is just a remnant. Nonetheless, the overall look is very elegant and rich.


I will attach the REW results as soon as possible. Thanks for checking out my speaker build! This has been a fun experiance!

[al bundy]

none of your attachments are showing up

[Mike P.]

al bundy wrote:

none of your attachments are showing up

I can see them just fine. Must of been a glitch.

Dan, what is the net volume of the cabinets?

[dan77]

3 Cubic feet. At least that is what the Rythmik directions say.

[Owen Bartley]

Dan, thanks for the very detailed build thread, and great work so far! I like how you did the bracing. Keep it up, and keep taking pictures along the way.

[dan77]

Thanks for the encouragement. I have been posting for days now and only moderators are making comments. Maybe its because I have been taking such wonderful pics that everyones questions are answered! I do see that alot of views are occuring. I Still have more pics, will post them tomorrow. Thanks again.

[Owen Bartley]

They are probably just overwhelmed! lol. Your step by step photos will be a great addition to the forum, its always helpful to see a few different approaches for first timers. They look like they're turning out great so far, and I really want to see the sandwash finish. We tried it a few years ago when I helped an aunt paint her house, and we messed it up quite badly, but it looks like you've got it under control.

Is it just me, or do the H-nuts seem to be in backwards... I know they will work just fine, but I would have thought you would insert them at the top of the leg (recessed) and screwed up into them, to get them to bite more as you tighten the feet. It shouldn't really matter since they're glued in anyway, but maybe its just because I'm posting too early this morning.

[tcarcio]

Very nice, Can't wait for some reports on the performance.

[jmuir]

Nice work Dan. I really like the look of the drivers. For me I would have them front facing to show those beauties off. Thanks for sharing!

[dan77]

Thanks jmuir, now that they have been completed for about 2 weeks, I agree. I ran REW against them and the only place in the room that sounds best for REW is on their sides aiming at the couch in a 45 degree formation. So the fronts fire right across from each other, like this: \/ The two speakers form a V. So now I have the feet staring at me. The wife does not like this at all. Not sure yet on what to do, still working on that detail at the moment.

[Owen Bartley]

Great job Dan. See, I told you that sandwash was trouble!

I'm with you on the tactile feel of bass as well, the Buttkicker LFE kit I'm using really does add to the experience, and if set up properly, doesn't distract.

[dan77]

Exactly, they can easily over power the subs, hard to believe, but they really can. I cross them over around 40 and turn them up only just enough to accentuate the subs. The effect is crazy and something I could never see not having in the future. It all started when I put a ButtKicker on my computer chair, where I have another 5.1 system with another 10 inch sub and the buttkicker makes you think you are really in the battlefield. When a tank rolls by you want to open the window to see if it is just outside the house. That is how real the effects become.

Oh yes, the sandwash was trouble. Especially since I had never used it before. But the finish seems so appropriate for the application. These puppies are so heavy that the rough surface is helpful as it lets you 'get a grip' on them when lifting.

[random username]

nice job on the subs. I made one about like that. My main problem, was the panel saw at Lowe's was rediculously inaccurate (they did warn me about it but...). Also, those plans are ok but I didn't look/think ahead....there are some missing dimensions and seems like something with the bracing didn't line up quite right, but ended up ok. I spent a couple evenings just tinkering with the bracing (which I glued together like a big "plug") to get it to fit nicely. Planed/sanded down the high spots and one or two areas I built up with veneer.

I believe if you calculate, your boxes are closer to 4 cubic feet.

I am wondering if more dampening material would help? I probably have the same amount as you, that's all i've ever tried.

Yes these subs aren't fun to move...but one way to help move from room to room is lay it on an old blanket (in your case tip it so the spikes aren't down of course) and drag it.

Regarding your grille...are you sure that you have enough clearance there? I would guess the cone and surround comes very close if not touching it, just judging by the photo. I believe Xmech (maximum mechanical excursion) for this drive is .9" one-way, so you could measure when the sub is off, to make sure there is enough clearance.

Rock on, brother!

[dan77]

Good idea on the blanket.

However, now I am going to have to run the calcs on the box size. The plans from Rythmik said 3 cubic feet, how could they be a whole cube off? Geez, now I gotta get a calculator and tape out after the fact. I would think that one whole cubic foot would pretty much ruin there ability to work well. I can't complain the way they are though, I get a nice bump at 21 hz and they do sound good.

Also, about the plans, I also think they could be a little more descriptive as well. You have to really think it through. All the drawings are there, but you have to extrapolate many of the dimensions. I did not mind so much. I think they were done that way so that you could choose any material thickness for the cabinet, and what dimensions they did give would not change.

Now geez, where is my measuring tape?

[random username]

My comments on the plans were not a knock on Rythmik by any means...I was just happy to have any plans to use. I should have looked ahead more. The only real problem was with the bracing, the solid areas between the cutouts didn't line up quite right but I made it work.

When I spoke with Brian I think he indicated four cubes was a good size for this setup but if we went much larger, the amp could overdrive the sub. The cabinet acts as somewhat of a limiting device. Larger volume equals more efficiency. Mine shows the hump near 20 Hz as well.

The mains I'm using are small nearfield monitors, and they start to drop off below about 120 Hz...so my next move will be larger mains.

[dan77]

Thats hilarious. That is what happened to me. I pretty much knew that I would need better mains once I got true subs. I emailed Brian about the option of crossing over at 100 hz (to help with my weak mains) and his answer was "a sub is a sub". Very true words. Thats when I knew I was moving into serious territory when I was purchasing them.
So anyway, my workaround is to use a 12 band EQ to 'bump' up the lower end of my mains. They are garage sale special 12 inch ported speakers, so they can take it. But overall, they lack in presence.

You know I am doing the math on 18 by 22.5 by 26.5 and allowing for a generous amount of baffling, I still come out with 4.7 cu ft. does that sound correct to you?
The measurements above are outside, so the inside dimensions are 16.5 by 21 by 25. So my math is, without the baffling is:
8662 cu inches / 1728 = 5 cu ft
Thus 5 cu ft minus some baffling = 4.7

[random username]

your method of calculating is correct but i'm not sure about your enclosure dimensions. The plans from the website that I used, were for a downfiring unit...so part of the overal dim. is accounting for the legs etc. I followed these plans but just made it front firing. Inside of mine would be 21 x 21.75 x 16.5, which is 4.36 minus driver, bracing etc puts me around 4.0.

My sub is in a "nearfield" orientation, I temporarily put it right next to where I sit so I could tweak knobs, and have left it there ever since. It is very nice to be able to make adjustments while listening. But it would probably sound better in the far corner or wall of the room. I usually use 80 Hz on the cross. switch and around 70-80 on the dial...and it is still localizable. If I put the crossover lower, it is less localizable but then there's really a gap in the response.

With mains, you want the response to be pretty low (capability wise) overall, so that it is really flat to say, 80 Hz. The ratings can be deceiving, because most are +-3 db but if the peak is right before it rolls of on the low end, the bottom of the rated response can be -6!

At least with your mains there is some paper there to work with. Mine are 5 1/2" woofers. I'm doing some boosting at around 85 Hz but we must be careful with that.

[Mika75]

What kind of spl are u reaching at the listening position, and at what Hz ?
(that driver does look sexy )

[substrate001]

I just wanted to say, great job! You obviously have a love for great sound, and you'll get it with those subs. I built just one that ended up weighing about 115 lbs, and the extension, speed and control were fantastic. I loved your approach to your home theater, as well.

[dan77]

You know, I have not tried to reach a max spl, do I do that with white noise or just a movie track? I am not sure which way gives me the proper result. I do know that I can overpower my listening experiance with bass and have to tune them down a bit. I like alot of bass, but I still want to hear the story line (sometimes, anyway)!

[bborzell]

Dan...

Thanks for the detailed account of your project. I think this will be quite helpful as I begin planning out a sub project, myself.

I will be using a Rythmik DS1200 and probably the 300 amp. That is what is in my F12 (stock build from Rythmik).

Will also go front firing.

[clausdk]

Very fine work!

Nice spike feet btw.

[dan77]

The REW results are in!

The following results were collected using a new Galaxy CM-140. I followed all the recommendations from this forum as to how to calibrate the sound card, graph mins and max's etc. So I believe all the data below to be as accurate as possible. There is no smoothing on any of the charts.

So here is the update to my build of two 15 inch sealed Rythmik subwoofers.

Location had an enormous affect on the output of both my old and new speakers. I used to move them around and find an ideal placement using my ears, but now with REW I have a more precise way to find the best location. So the following is my journey into trying dozens of speaker locations and settings in order to get the best sound possible. In the end I believe I have done that. But you be the judge, the charts below are the most meaningful ones that I recorded.

For this charting exercise, I started out with my original home made subs of 15 years ago. I charted them for comparison purposes and to see exactly what I had been listening to for the last 15 years! They are 12 inch down firing ported Rockford Fosgates (car subs) tuned to 45 Hz. It looks like they are tuned to 200 Hz though!
It's amazing how much bass I thought they were able to create throughout the years.
Here I try two locations to see how they performed and to use these charts as my baseline.
Old subs directly in front of the screen

Old subs in the front corners of the room. This produced a noticeable dip around 77 Hz.

For the new Rythmiks 15 inch sealed speakers, I played around with many a location. Behind the couch, other end of the basement (about 45 feet from the screen). Near the water heater (or about 10 feet behind the couch), etc. These extreme locations produced severe drops in certain frequencies. Below are the locations that seemed to work fairly well. My final location and orientation is at the bottom. Orientation seemed to have a pretty large affect on frequency dips, not so much output. So my goal was to find the location with the least amount of 'dips' or 'peaks'.
My basement is unfinished with black felt hanging from the ceiling along with a photographers muslin behind the front screen. I placed a carpet remnant on the floor to get rid of the cold concrete. The entire basement is unfinished. Here is a drawing of my basement layout. The area with the couch and screen is 16 foot by 21 foot, but opens up to the remainder of the basement. There are no walls. The entire basement has 9 foot ceilings.

Here I placed the Rythmiks on each side of the couch, and a little forward (in other words, center of room on each side), facing down. Notice the wretched drop at 32 Hz and the overall lack of bass being produced.

Another location tried is with the speakers in the extreme front two corners of the room. Orientation made a big difference here. So the first shot is with them both facing away from the screen. Dips and peaks everyhere! Run!!

Here they are both facing in the same direction (east). Definately better, but noticeable drop off above 60 Hz is introduced.

Here they are both facing the sky. 28 Hz to 38 Hz frequencies are lost with this configuration.

Next I tried both speakers directly in front of the screen.
I tried them each 'facing' each other in a 45 degree pattern as shown below in picture:

Why did I try a 45 degree angle? Because when I had them both face me directly or to the sky, down or any other way in any other direction the dips became extreme. I must have tried over 2 dozen variations of placing them in front of the screen. In the end this 45 degree angle produced the best results. So here is the chart from the above picture.
As you can see, I get bad drops at 32, 48 and 64 Hz.

Same orientation, I just pointed them to the sky. Oops, there goes Mr. 65 Hz!

So in the end, here is my final location (Old subs not hooked up for picture or REW testing). Best results I could find is with the speakers located one in a corner facing east (perpendicular to screen) and on in front of the screen dead center facing me as shown. This final configuration took three 4 hour sessions of testing on several different evenings in dozens of locations to come up with which seems to produce the best sound possible.

Here is the chart. No crossover or rumble filter.

Here is the speaker location but the rumble filter is turned on and the crossover on the speaker and the receiver are set to 80 Hz, but fall off does not happen until around 150 Hz. Don't know why. I also have the PEQ set to plus 3db at 25 Hz. This causes both ends to fall off and the hump between 35 and 40 Hz to be more pronounced. Can't really hear the difference during a movie scene, but the rumble filter is supposed to help prevent damage to the speaker during loud scenes.

Here is the final shot with my mains included. The reciever and subs are set at 80 Hz crossover. I purposefully have the high end turned down, but the rather noticeable drops in volume around 48 Hz and 150 Hz and 500 Hz I could not get rid of using my cheesy 12 band $10 equalizer that is about 15 years old. Also, my mains are hanging from the ceiling, so I cannot move them around to see what affect that may have.
Does anyone know how I can improve the 48 and 150 Hz drop? It does not exist with the Subs alone, as shown above. It only occurs when I combine the subs with the mains. 150 Hz is lower end of voices, so I would like to level that out.
Final location, orientation and settings:

Here are the main speakers with no subs, so you can see that the 48 Hz drop is only from when I combine the fronts with the subs. Is there a way to manage this negative harmonic interaction?

[paulspencer]

Dan,

Thanks for sharing your journey! Some very interesting results.

I'm not sure if you're still after suggestions but I have a few. It looks like you have tried options with the subs placed together, and you have measured by moving them. Here's an alternative. Place one sub in the listening position, then move your mic. Measure all over the room, including different heights. Even if you can't use the locations, it's educational!

Instead of just a few measurements, it's easy to take 50 then you get more data and can come closer to the ideal. It will get you there quicker.

Then I suggest you place them differently, choosing locations that give you synergy. Where one sub has a dip, the other should be placed so that it doesn't. The combined response will ignore the dip, so you aim as a minimum to overcome the dips acoustically and try to minimise the peaks, but the ones you can't remove can be tamed with EQ. Once you have your data you can narrow it down, then start measuring with the subs in those places. From here you can also experiment with phase settings. You might also try some downfiring options again. When I measured the corners in my room, the best response was right down at floor level and ceiling level.

You might also consider building some bass traps.