Sub recommendations for HT


What are some good subwoofers in the $500 - $800 price range for a large room. I'd like to keep the sub itself as small as possible though. I have considered the Sunfire subs but they are a little out of my price range.

Thanks for any help.
snicka
Thanks for the input.

I have been looking at Velodyne ans Sunfire in the used market, but have not made a decision yet.
If you haven't purchased a sub yet, what I gathered from talking to Mike/owner of ACI, is their 'Force' sub, $750 is the same sound quality as the Titan II with lower output. I was willing to spend on the Titan, but Mike suggested the 'Force' for my lower output needs. I'll be using the sub mostly for music and the occasional movie.

Side note. I've never known of a more customer friendly company.