My first thought would be that the back wall would be fine. But if It were my room before mounting them perminantly I'd experiment. Play the speaker (as a main) and see if you can pin point it (i.e. you want the sound to come from generally the rear/side R or L but not from the speaker itself). The further you can get them from directly over the couch the better IMO.

To do this I'd suggest creating a temporary stand out of any scrape wood you have lying around (at worst by some 2x3') to hold one QS10.

Here's a pict of the one I threw together (took about 30 mins total).
[Linked Image from i.postimg.cc]

Just an inverted T which can be leaned against a wall. Either put something heavy in front of it to keep it from falling forward (the couch, a table, ... ) or screw on a third leg ... it need not look pretty ... the scrape piece of strapping I used is not even straight!

[Linked Image from i.postimg.cc]

Then try different places and see what you think.