If you are looking for a well priced sealed design, a good servo mechanism makes a big difference in performance. The Rythmic and Paradigm Servo are good examples and even the more expensive Velo DDs use a digital servo control. IF you don't mind doing some DIY stuff the Rythmic subs are pretty cheap for their quality and excellent servo technology. Servos add alot for not much extra cost.
I had an opportunity to listen to a pair of JL F113s "super subs" a couple of nights ago and they were very impressive, powerfully tight sealed subs with no servo. Instead they have 2500W amps and a massive 70lb driver but they retail for over US$3000 each.
