I ended up getting an 8" 80 watt sub from Dayton Audio on a flash sale for $60. It is perfect for music in the small room I am putting it in. I will admit that it is a bit boomy, but if I turn the crossover as low as it will go, it blends pretty good with my bookshelf speakers. Can't beat it for $63.60 shipped!
-Geoff
Try turning the crossover UP to 80-100 hertz, it should make it less boomy and help the midrange of your bookshelf speakers.