It's really hard to answer that question without trying. So give it a try. It's not going to cause any physical harm putting the sub behind the couch. It just may ruin the illusion of the bass coming from the screen, and localize it behind you. Only one way to know.

Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs
-Chris