it's simple.
connect the line level outputs for L/R from the processor to one set of pre-amps inputs. connect all the two-channel sources to the pre-amp. connect the pre-amps L/R line level outputs to the two main amplifier channels. connect the processors other outputs (Center/RR/LR) directly to the surround amplifier channels.
then there isn't a compromise for 2 channel sources.
connect the line level outputs for L/R from the processor to one set of pre-amps inputs. connect all the two-channel sources to the pre-amp. connect the pre-amps L/R line level outputs to the two main amplifier channels. connect the processors other outputs (Center/RR/LR) directly to the surround amplifier channels.
then there isn't a compromise for 2 channel sources.