Instead of buying the retail subwoofer, you should strongly consider putting together one. I don't mean actual DIY. You can purchase a pre-made/pre-finished sealed cabinet. You only need to cut a couple of holes in it with a jig saw. If you don't have a jig saw, you can buy one cheap, for about $20-$30. You would use a 3 cubic foot cabinet from
www.partsexpress.com, an Eclipse SW8200 driver and an O Audio BASH500 watt plate amplifier. This solution costs about $700-$800 total and will exceed the quality of any retail subwoofer for the same price. This is due to the extraordinary driver I have specified which uses the LMS technology from TC Sounds/Audiopluse, the designers of the highest quality subwoofer drivers in the world (measurably). If you were to actually consider building your own cabinet, I can recommend other options that can exceed even this in performance (as you are limited to a sealed cabinet for the pre-made cabinet system).
-Chris