If you are not using the system for movies, you generally don't need a deep digging subwoofer. Most music seldom digs below 40 Hz. The Monolith 10" would be good. Others to look at are the Hsu VTF-1 mk3, SVS PB-1000 or SB-1000, and RSL Speedwoofer 10. The nice thing about the SVS subs for this application is that they have high-pass filtered RCA outputs, which is perfect for a desktop powered monitor system. It is some basic bass management, so the speakers get everything about 80 Hz and the sub gets everything below 80 Hz. As for lower cost subs, I would be looking at Dayton Audio and Monoprice for that.