I's suggest that if your couch can't be moved out from the back wall, then you won't get a lot of improvement from going 7.1. Placement is also a big challenge. I tried it as an experiment with just a few feet to spare and I ended up going back to just 5.1.

You are not missing a lot as so little is even recorded in a true 7.1 soundtrack. It will get more common with Hi Def but I wouldn't worry about it.

With great power comes Awesome irresponsibility.