As I concur with the above advice I would suggets the Vandersteen Sub which used should run $750.It has an execllent crossover which is is accessed through a direct hook up to your speaker posts and responds to the full range signal not just what is low passed.Very clean and quick do also to fact that it uses multiple 8" drivers.Call a sub manufacturer and they will tell you that bigger is better but a large cone and magnet assembly does not always move fast enough to reproduce pitch accurately.If it were Home Theatre thast would be a different story where bigger is better for that "boom in the room".I vote for Vandersteen and if you can afford it the REL Strata III (starting with one and getting second).But all mentioned above are good.You didn't say what kind of assciated equipment you had so all of us are being presumptuous and as you will have noticed freaks.But as it was mentioned before a really cheapo sub good for HT will screw up decent sound.If you are looking try to find one with smaller faster cone and get two but also try to look for one with a servo amp which will produce less distortion than a non-servo.
G'luck
G'luck