Adding a sub to a 2 channel stereo?

Looking at HSU Research for a decent sub, not looking to break the bank here just improve the overall bass of my 2 channel with tube gear.

Any other good recommendations?
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.
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.
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 second the sealed sub suggestion. Hsu Research will be fine for any application -- it's a matter of setup. I'd also suggest M&K; their push-pull driver configuration is said to cancel 2nd harmonic distortion. Don't forget the bass management controller.
HSU is a good start.