Bayne, as I suggested previously, hopefully the back surrounds could be separated widely enough so that a listener could experience both a left rear and right rear effect on material which intends this(assisted by DPLIIx or Logic 7 processing). The separation that you describe would be enough for listeners near the center, but if you go with ceiling mounts it would appear that you could accommodate a somewhat wider separation.

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Enjoy the music, not the equipment.