I've gone around and around on this sub question -- I'm running a 2.1 setup with the FS52 towers. I've looked at all the sub $300 options (Monoprice, BIC F12, Klipsch RW12D, etc.) and I'm just not that impressed with any of them. I've decided I'm going to wait a while, try to be happy with my little Audiosource SW80, and save for a HSU STF-2.
Of course, that doesn't stop me from monitoring Craiglist in the meantime...