Mark, you didn't specifically give a key measurement, i.e. how far the listening position is from the back wall, but I gather that it's fairly close. If so, the common advice applies: putting them about 3' above ear level, either on the back wall spread out almost to the side walls, or the side walls almost at the back wall. It should make little difference. Side surrounds in a 5.1 setup should preferably be slightly farther back than directly to the side, rather than toward the front.

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Enjoy the music, not the equipment.