Griff, the problem isn't the mounting on the back wall or the height, which is well within the recommended limits. The separation appears to be far too narrow. The surrounds in a 5.1 setup are side surrounds, even when mounted on the back wall, and should be separated as widely as possible, maybe just a few inches from the side walls, so as to give more of a side, rather than back, effect.

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Enjoy the music, not the equipment.