I did basically the same thing using a 750 (the 755's predecessor) and a 1050. The 755 can be used to drive the fronts, center, and surrounds (just connect the 1050's respective pre-amp outputs to the inputs on the 755), while the 1050's internal amplification is still used to drive the surround back speaker (just leave the surround back speaker connected directly to the 1050). With no other channels driven, I think the rated output for the surround back channel goes up to around 90W. I used that arrangement successfully for several months before adding a used two-channel amp for the surround back speaker and replacing the 1050 with a 950.

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