Sub options - good used, compact, sub

Any recommendations for a decent compact sub for a HT system that is often found on the used market for under 400? The only one I could find that looked promising is the AV123 Onix but I can't find any used ones at this time.

Look to Velodyne. I've always had great luck with them. Some are priced a little high but even the sub $500 models will shake your house without exposing where its hiding.
Hsu Research packs a lot for a low price. There's one for sale on audiogon right now.
Hsu or Svs...I prefer the latter. Nothing touches it for the price.