I think you would be better off getting two 12" from monolith than one big 15" from them. Two subs will give you a better in room response across the listener area but this will also requires a purchase of a miniDSP 2x4HD, buying a measurement microphone such as a UMIK1, and downloading REW onto your laptop and using it. Its a learning curve that I went through but its well worth it. If you have extra cash, then buy a pair of those 15" beasts
but you will still need REW, a miniDSP and a measurement mic to dial them in properly.