Chris, welcome. There should be some distance open behind the listener and the back surround speakers to give the back surround effect a chance to develop. If I followed your description, you'll have about 4', which should be adequate. The 6' height should be fine and the separation should be wide enough so that anyone on the couch has a back speaker on each side of them(for directional effect), e.g. if the couch is 6' wide, the speakers might be separated about 8'.

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Enjoy the music, not the equipment.