What type of sub you need depends on what your source material. If you just want booms and explosions from movies can get away with a less expensive sub than if you are looking for full range music (or a system that does both).

Sone of the more cost effective subs out there are made by HSU Research. Like a Outlaw they are also a Internet/Mail Order operation. I haven't checked their line as of late, but they used to have some models that were unpowered (so you could use your amp). Their subs are generally reviewed very well.

Other companies that I've heard and auditioned are Audio Concepts (www.audioc.com) and Paradigm. I have an Audio Concepts Titan and it is a great match to my Magnapan MG-10's. Since they build to order I also was able to get the cabinet unfinished and then stain it so it matches my "Maggies". I've heard Paradigm's subs in a friend's home theatre setup and they seem to do everything pretty well.

Eric