Crystal Acoustics TX-12SU Subwoofer

Here's a THX-certified (we don't hear about THX anymore) subwoofer that goes down to 15hz.  Its $749. Not bad.

http://www.hometheaterhifi.com/subwoofers-products-menu-column1-41/1459-crystal-acoustics-tx-12sub-thx-ultra2-certified-subwoofer.html