Just ask Mark
2 subs or 3 or 4 give better, smoother bass response than a single. So as long as you can afford the floor space and have the dollars to buy multiple subs, this is the way to go no matter what the room size. Obviously in smaller rooms you can use smaller subs but you still need one really good sub(somewhat large - usually) to hit those really low notes.
Forgot to add to look for Paradigm DSP3200 or 3400 to add to your quest for dual subs, they are priced right and sound great, IMO, very similar to Axiom.