Ok... If you follow the Welti/Devantier method your subs need to be of equal capability. If you follow the Geddes method you start with a fully capable sub corner positioned, then an equal sub on an oposing wall, and third..somewhere else in the room, of what we assume is of lesser capability.
Now, if you stray from either like i did and achieved great results ( other than my subs lack output capability ) they should all be of equal capability. I'm sure there are other methods just waiting to be discovered, but at this point we are stuck with the two i mentioned , and my near field subs.
WTS, ^^^^^^^ +1 on the 15" driver recommendation, and the amp for what you can get used could be a pro amp.
Now i used the Dayton Audio RSS315HF-4 12" Reference HF Subwoofer 4 Ohm and the, Dayton 240watt plate amp. I'm positive the plate amp is causing the output issue, wont know till i pick up another amp like a used Behringer EP2500 used cheap. Check out the thread in my sig, the multisub one for picks and graphs and stuff....
As of Jan 2017, im still looking for (1) Outlaw M-200 mono amp, and a used Marantz 7010