Edge Audio Hyperion 12 Super performance for $599US. Buy it direct.
HSU Great reputation for quality bang (or boom) for the buck - buy them factory direct
Vandersteen Three 8" drivers in this sub make it a design that we really like. Buy it from their dealer base.
Paradigm Buy any Paradigm sub over $600 and you have excellent performance guaranteed and great value. Buy it through their dealer base.
Atak subs - simple cabinets, killer drivers and cheap enough to go with a distributed subwoofer system.