Mark, welcome. Your proposal of about 3' behind the couch and 8' up should work very well. As far as the separation of the QS8s, keep in mind that although you used the term "rears", the surround speakers in a 5.1 setup are side, not rear, surrounds. You should therefore separate them as widely as possible(i.e. not over the archway), so as to give more "side" effect, despite the fact that they'll be on the back wall(again though, 3' back is quite acceptable).

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Enjoy the music, not the equipment.