I use two Velodyne FSR 15" subwoofers, but it's strictly for extending the response of my main speakers, which fall off below 35hz. With my two subwoofers engaged the response is now 14hz-40khz, with a Max SPL of about ~108db or so in room.

The woofers are set inboard of the mains, and are approximately 6' apart center to center, and about 3' in from the front wall. This is to try to match the woofer plane with the mains. In this position I have found that the standing waves are much reduced from a corner placement. The output is also slightly reduced in this placement, but that is not a problem since there is so much unused gain.

I have used my dual subs in a HT experiment. LF effects were quite pronounced, (read earthshaking) but I didn't feel it was absolutely necessary, or "better" to have two subwoofers for effects, as much as I like them for the audio. To quality this; My HT mains, CSW T500's, are capable of prodigious bass, and although there is more distortion compared to the Velodynes, I didn't feel it detracted from the LF effects at all.

In my view, if you have the room, you can't go wrong with two subs.