No reason you couldn't use a 770 -- it would be just like using the 755 (since the 750 is now only available used), just with the extra two channels. You could split the surround back channel into two and let the 770 drive two surround back speakers, or you can use it for 6.1 and just leave one channel idle for now (since you already have enough speakers for 6.1) and have it available down the road if you ever need it.

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