Best sub for HT?

I just got a Pioneer 1120-k as a black Friday present for myself, and I'd like to get a good sub for HT exclusively. I have an old Sony that frequently gets over driven during movie watching. I'm looking for the best bank for the buck sub. My current setup is a 5.1 system - Infinity SM82 and Video1 center, with Mirage rears.

Thanks,
Mark