Adding a sub to a 2 channel stereo?

HSU is a good start.

I've owned three subs over the last 8 years:
- Atlantic Technologies 352PBM
- Velodyne SPL-1000
- REL Storm III

The REL blows the other two away, and I don't foresee the need to upgrade... unless I run into a lot of fun money, in which case, I'd just move further up the REL foodchain.

I would look into sealed subs if possible, HSU is a great brand for the buck and I owned one for a couple years but it may be too slow for a music application.

There's a ton of responses to this exact question over the past several months. Suggest you search on it - you'll find what you need. For people on a budget you will generally find support for Hsu, ACI, cheaper RELs and Velodynes among others.