The short answer to your question is no. One sub that is powerful enough to match your other amplifier output and keep up with your room size will work just fine. This being said, I personally am a big fan of multiple subs. Two smaller subs in medium to large rooms can have some great advantages from the point of view of getting just the right amount of bass at all the various listening positions. It takes a bit of experimentation to get the sub placements and volume settings correct in your particular room, but it really works well. The other benefit, which can also be achieved by just having a sub that is bigger than you really need, is that the source material is not all recorded the same and some times you need to turn up the sub to get a proper balance of the low frequencies to the rest of your system. This is especially true with some Home Theater recordings and much less so with stereo.
