I used an HSU VTF-2 in the configuration you're describing, behind the couch, but with less space between it and the back wall than you're describing. It worked extremely well in my space. Give it a try. The sub fit perfectly underneath a very narrow, long table that was just the height of the back of the couch. I can't use that setup where I now live but I preferred it to my current configuration.
Sub usefulness in constrained placement situation
Hi - I am considering a sub for my system (2 channel & HT). The problem is the furniture/room design means that possibly the only place to put it would be behind the listening positions (couch) where it would have maybe 4' - 5' between couch and wall. Would that degrade the performance ? I was thinking of one of the room adjustable subs - Velo DD or Revel, to help. Any thoughts appreciated