Also for right around 300 bucks if you have time you can create your own sub. Ive got picks of one i did that would blow away most stuff. 600watts,Polk 12inch sub, max movement total from top to bottom is like 2 1/2 inchs at 18 hz, and its about the size of your 15 inch monitor but weighs about 40 pounds.
http://www.free-image-hosting.org/pic.php?pic=6/17400574571.jpg
http://www.free-image-hosting.org/pic.php?pic=6/17400570357.jpg