Right, MM, the idea of side surrounds is to surround mainly from the side, not from almost directly behind you. It's suggested to have the surrounds on the side wall about three feet above and two feet behind ear level. Since you say that you can't do sidewall placement, the next best thing is to get it as close to the above as possible by placing them on the back wall about three feet above ear level and as widely separated as possible so that they're off to the side.

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Enjoy the music, not the equipment.