Originally Posted by duc135
Equipment list: [shorthand added]
Denon AVR-4311 receiver
Revel F52 tower speakers - connected to Emotiva XPA-2 amp
DIY sealed LMS ULtra 5400 sub x2 - connected to QSC 5050 amp [S1]
Revel B15a sub - active sub [S2]
DIY JL Audio 10w7 sealed sub x2 - connected to Behringer EPX4000 amp [S3]
Option 1 - get two balanced 2x4 units. One for the two or possibly three subs and the other for the L/R speakers and MBM integration.
Option 2 - get one balanced 8x8 unit to do all the above.
Dude, you're making this way too complicated, and IMO over processing. And introducing too many possibilities for noise to get into your mains.
First thing, I highly doubt the 10W7's will make competent "MBM's." Too much inductance, so they don't go high at all. They're spectacular small deep bass pumps - great suspension linearity, class-leading motor linearity - but they're very much one-trick ponies. Your other two subs are much better performers in the 80-120Hz range.
Second, pretend you only have one sub out. Audyssey handles multiple subwoofers so badly, that the better course of action is to merely pretend it doesn't have any multisub capabilities at all.
Third, your mains are good enough that you shouldn't need any processing to them beyond what Audyssey provides. Though you may want to but an Audyssey Pro license and buy/borrow a Pro kit, so you can eliminate that infernal Audyssey "crappy speaker compensation" midrange notch.
What I'd recommend is just buying one balanced 2x4 miniDSP, hooking it between the AVR and the three subs. If you don't have already measurement gear, spend the balance on FuzzMeasure Pro, an ART USB Dual Pre, a tripod, and a calibrated mike from Cross Spectrum Labs. (Or Omnimic from PE, if you're a not-a-Mac guy.) If you do, buy tickets to musical performances, or recordings, with the surplus disposable income.
What I would do is take your mains, run them in LFE+Main mode, set the sub crossover in the AVR to maybe 120-150Hz, and follow Geddes' advice as to sub calibration. Start with the mains running, then fold in S1, then S2, and lastly just use the S3 to increase extreme ULF (lowpass at maybe 40-50Hz).