I would look at these:
HSU STF-2 - $370 ($320 +50SH) This is a real deal subwoofer and it's on sale right now.
RSL Speedwoofer 10S - $400 This is supposed to be a very good subwoofer in a smallish package.
Dayton Sub 1500 $200 It's just a big box with a big driver.