Check out the LAT from Tymphany...bizarre sub, but it works REALLY well and sounds good too. Look up "LAT tymphany" in google and it should be the first link. We have a couple that we have been playing with and it is really promising. Alpine also sells it in their new bandpass box.

Another alternative is the Peerless XLS 10 inch...we put them in to a box that is only 3.7" deep and it sounds great!

You could also go with the Elemental Designs EU700 6" sub...it beats hard, plays ungodly low, has 13mm of xmax each way, and is fast and transient. Not to mention it is only about $50 each. There is a guy with 15 of them in the floor of his matrix in the spare tire well and he competes with it.

Good luck!