Brian, of course those two mounting positions aren't alternatives. If you wanted to first have side surrounds for a 5.1 setup, placing them to the side but maybe 3' forward would be usable, but directly to the side or slightly farther(maybe 2')back is better. Then a center back speaker could be added for 6.1 and that middle rear sound field, but two back speakers, separated adequately, could form a more continuous 7.1 field.

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Enjoy the music, not the equipment.