I think you'll run into the problem of when is it enough. I currently own dual PB2000s in a 1900 cu ft room and although it is nice, it isn't anywhere near what I know it could sound like. I used to have a JL 13w7 with 1400W RMS in a tiny car and when it hit the low notes, your eyes vibrated and your face felt like it was rippling. It was a very unusual experience but a very awesome one. Now do you need that in your home theater? Probably not, but it goes to show that there is always something more.
Have you taken any measurements of your subwoofers? If so, could you post those so members here have an idea on what your current subs are capable of in your room.
Denon X4000 | Paradigm Studio 60s | Paradigm Studio CC-590 | Dual SVS PB-2000s | Panasonic GT50