The only good small sub is by Sunfire or Pinnacle but the SVS or HSU outperforms them in sound and price.
http://www.hometheaterhifi.com/volume_9_1/sunfire-subwoofer-junior-2-2002.html

http://www.pinnaclespeakers.com/rvubboom.htm


http://www.quadloft.com/hometheater- new HT