The Vandersteen 2Wq sub is one of the best values in subs on the market, selling new for about $1300,and available on the used market for $800-900. It features adjustable "Q" (damping), as well as adjustable sensitivity, so it will match well with most dynamic speakers. Thiel speakers are, like Vandersteens's, both time-coherent and phase-accurate, so the Vandy 2Wq should work well with them. If you intend to use the sub mainly for music reproduction, the 2Wq is your best bet, but if you will be using the sub mainly for home theater, then you might want to buy the Vandy V2W, which has 6db higher output than the 2Wq. As most of the 2Wq owners who post on this site will agree, a stereo pair of 2Wq's is much preferable to a single unit.
The other moderately priced subs that are worthy of consideration are those made by Paradigm, PSB, Hsu, and REL (the Stadium and the Q201E).
The other moderately priced subs that are worthy of consideration are those made by Paradigm, PSB, Hsu, and REL (the Stadium and the Q201E).