A good choice for a sub for music would be the Outlaw LFM-1. You can get a pair of them for $999 not including shipping.

Wid posted a link in this thread a few days ago to a nice write up on using a pair of subs for stereo.

I have an LFM-1 and it produces nice tight bass. Wish I had the cash at the time I bought it to have gone for the pair.