Diy sub

Hi, Please describe in detail what you intend to do, what you have and what you aim to achieve so I can see if and how I can be of assistance.

I have a crown xls1502 to power them. They. Are 1000 watts rms a piece. I have 2 of them,

I hope I simmed it right on this quick one. It seems you will do well with each of those in a 3.5cuft(net) ported box tuned to 20hz. It looks like each can hit 112hz at 20hz with 1000 watts with a 3rd order highpass at 18hz. Two round ports 3.75 inch diameter and port length a little over 39 inches are needed. It's a compromise with many parameters at the border line but if I'm right you'll get it good down to 17hz.

So I. Can tune them lower than. There. Fs can I ask. How. You figured this out. Can I use. 4 in pvc

And. I only have the ability to. Do. 30hz high pass or I can. Do. BANDPASS how can I do. A 15hz 3rd order

Check out WinISD.
3.75 inch pipe is the absolute maximum port diameter. Any bigger and the port gets long enough for its pipe resonance to distort the sub's sound.
I have attached the spl graph here from WinISD to see how the 20hz tuning was done.
The 30hz highpass on your amp is not useful for home theatre subs. Disable it and get a loudspeaker management system, put it before the amp to implement the 18hz 3rd order highpass (not 15hz) as well as a 4th order low pass which you will need at about 80hz..

Like a minidsp and 3.75 id pipe

You got it.

Should I get the. Dsp with. The. Mic or just the. Dsp

Or like a dbx drive rack

I don't have enough experience with these DSPS perhaps someone else might chime in but my understanding is that minidsp has distinct advantages and with the mic you not only implement the filters but also measure, set up and eq your entire system.
I actually would have anticipated that you would be asking more about the details of building the boxes. Perhaps you've understood that aspect very clearly.

The box sounds pretty straight forward or is there something I am missing I know. My sub takes up .14cuft3 I will need to factor that. In building the. Box and some braces inside. Prob round dowel good glue screws and any advice you have. I just didn't know how to. Do. The 15hz high pass

I just didn't know how to. Do. The 15hz high pass

It's 18hz not 15. You just get the dsp you want (minidsp, the higher types of dbx driverack, behringer dcx and I think rane dsp) and you simply key in the settings for 3rd order 18hz highpass and you're done.

Yeah. I will build. 2 boxers @ 3.5 net after braces speaker displacement idint know I needed flare ports but I can do that should I do. Front ports or back or build it like. A svs short square but deep. Do I need to. Dampen the inside of the. Box with polyfil or foam

I'm not sure if your reference to svs port is a consideration of changing to a slot /square port. If you do that you must take care that the slot port surface area equals the sum of the surface areas of the two 3.75 in diameter ports and the port length will need to be re-simulated.

If the intended net volume has been accurately implemented, all you need is 1.5 inch acoustic foam lining the internal walls of the box to kill resonances.