Kioko,
I own a PB12+/2 and love it. I upgraded from an M&K sealed 12" and I've worked over the BFD with REW (4 filters).
Forgive the long post, but maybe something below will be helpful.
I, too, was at a brother's house last weekend and astounded at his $500 Sam's Club 5.1 HTIB bass (it was corner-loaded). Funny how I worried slightly the same way as you about bang-to-$ ratios!
Yet, upon returning home and cranking mine the smile returned and I appreciated the lack of "one-note" boom--don't get me wrong, it booms, it just does all sorts of other things also.
There's a great scene in The Island (chapter 12 I believe) where there are all sorts of different frequencies at play. Some are upper-chest gun shots and others are infrasonic shudders. It's a terrific sub demo as many lesser subs can't do the full spectrum of LFE. You should try it--I've never found a better all around demo scene for every kind of LFE...
I had the cool opportunity to visit Ed Mullin's home (of SVS fame) this summer and heard his dual Ultra 12+/2s (in a near-field setup). He has a pretty flat system setup, and we used all three different tuning points on the Flight of the Phoenix plane crash. Holy Cow--it was unreal.
I've heard a lot of things in my days (used to be a mobile dj), but that takes the cake! My pants and shirt sleeves were flapping--but the volume audibly wasn't loud at all. It was eerie and I loved it.
The most impressive thing was that I went home and tried my own single +/2 and it couldn't do it at all--oh, it could pull all the booms, etc. of the later crash, but the wind-flapping stuff prior to the crash, etc. were not there.
Ed told me that the dual subs allow for basically double the headroom, so they don't over-extend, but can go lower with less effort. I think the distortion must be greatly reduced also. I'm telling you, the infrasonics on Ed's system were so cool. In fairness to my +/2 it is in about a double-sized room as Ed's and there's just the one box! Still, I was so impressed!!! It's got me thinking about a dual sub setup big time.
Now, I love my +/2, but if I could get another one (or a PB13) to compliment it, I'd be loving life for sure.
I'll write back in a couple days after I've been able to a/b my +/2 against the F113--I'm putting the JL into a buddy's house and will be hauling my SVS over to his room to see what they both sound like.
Seems to me you should ask yourself some questions about what type of sounds you like. If you like the subs hot, ask yourself if you care whether there's a lot of accuracy or if you just want to get 'hammered'? Car audio is infamous for its distortion--it can be disconcerting to hear "accurate" bass coming from that experience.
Would it be possible to take a different sub over to the showroom where you heard the JL and audition the room? Or would they be willing to throw some of their other subs into that same room/position to a/b the JL? That kind of demo is likely to help you out...
I know the SVS guys are truly helpful as per advice--just email or call them and you'll get steered right.
Enjoy the search...
Phil