An added benefit of going with multiple subs is that you gain extra output in comparison to one sub, if they are placed well. Only people with enormous rooms would need to buy extra subs in order to get sufficient output at reasonable listening levels. After all, Axiom, Hsu, SVS, Rythmik all make subs that will be able to give good bass output in very large rooms.
It's not so much the output level; One EP500 set at 5:30 or 6:00 (v2) was enough to rattle pictures on the wall and make a closet door behave like someone was trying to get in (19' x 21' x 8'). It's about balancing out the nodes in the rest of the room - that's what the extra subs get you.
