Why can't you use your amp in surround mode? If you're using the AVR as pre, it should work as is, other than maybe adjusting front channel levels.
Actually, you're partly right. I can get the front signal from my AVR to the amp and then the frnt speakers do work. However, they are out of sync with the "surround mode" as the power amp "over-drives" the front speakers. To get the accurate digital info (I think) I need to be wired to "front speakers" on the AVR. It is the AVR that "mixes" the signal.