Personally, I think a lot of people on AH think they need more sub than they do. 4000cuft is not that big. But if you insist on total overkill it would be hard to beat a pair of the Monolith THX15 ported subs, assuming you can put up with two huge, heavy, and IMO ugly boxes in your room. If you haven't read it already, James Larson posted a really nice review of them here recently:
At last! It's here! Our review of Monoprice's monster subwoofer, the Monolith 15" THX Ultra! This 130 lb. beast might look like it's all muscle, but there's at least as much brains as there is brawn.
www.audioholics.com
ps - I wouldn't get hung up on driver size.