You could always split the pre-amp out signal and power two channels from the single surround back output, or check out the
manual -- there's a section on using the 1050 to drive two surround back speakers.
Obviously, this means that both speakers (6 and 7) are getting the same signal, but currently that is true anyway of 7.1 decoding for most everything (Lexicon's Logic 7 being one of the only exceptions, I believe, but that's in a somewhat different league than the 1050 or 787).
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Gonk