Spence, I think that I've followed your description and picture and the only thing that I'd suggest would be a bit wider separation for the back surrounds. If they'll be about 13' behind the listening area, they could well be separated by a similar 12-13', to allow for situations in which they'll be carrying different sounds(e.g. through use of DPLIIx on 2 or 5 channel material), and so that there's enough separation for the difference to register at the listening area.

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Enjoy the music, not the equipment.