I am in the same boat- looking for a seriuos sub to go in about 2-3 ft3. Right now, the best candidate I have found is a 15" TC9 for about $175 direct from TC Sounds (they have other sizes as well). This model is the basis for the Eclipse Ti subs and is an upgrade to the Styke AV series which is based on the TC2. I have talked to a few of their tech guys and they have assured me that with 600-1000 watts, a TC9 15" in sealed 2 ft3 box will have amazing output and a serious low end.
Originally, I wanted to go ported, but the box was just too big. Not to sound loco, but the 15" TC9 in a sealed box with 1000 watts should get insanely loud, right?
If anyone hears or knows of a better option, please let me and the monkey know.