Mike, if the back of the couch is only about 4' from the wall, then mounting both QS8s on the back wall should give good results even though they're side, not back, surrounds. Separate the QS8 that's closer to the side wall as much as possible(say 6" from the side wall)and the other one a similar distance to the opposite side of the couch.

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Enjoy the music, not the equipment.