A good sub is a universally usable component. Buying a cheap farter will only turn you off. A sub/s with good EQ will blend seamlessly and resale easier if need be.
http://app.audiogon.com/listings/subwoofers-velodyne-dd-12-digital-drive-subwoofer-practically-perfect-condition-2013-12-09-home-theater-90266-manhattan-beach-ca
http://app.audiogon.com/listings/subwoofers-velodyne-dd-12-digital-drive-subwoofer-practically-perfect-condition-2013-12-09-home-theater-90266-manhattan-beach-ca