Hi Laura,
You would connect the Denon's pre-outs for the left and right front channels to the Yamaha's "amp in" for the left and right front channels if it has an "amp-in" L and R inputs.
If it doesn't just use any line-level analog input labeled "CD" or "Aux" or "Tape". Don't use "Phono" if it has that input.
Connect your front M80s to the Yamaha's L and R main channels and leave the Denon to drive the other speakers. Switch the Yamaha to Stereo mode and "Direct" or just Stereo is fine. Set the Yamaha volume to about the 11 o'clock position, which should give you enough output to hear the output from the Denon's front L and R pre-outs.
If you do hear the Denon test signal from the M80s, then you'll know that one of the Denon's main amp channels is defective. If you don't hear the test signal, then the Denon's preamp section has been damaged.
Turn on the Denon receiver first, then the Yamaha. Set the Denon volume control where you would normally set it so the pre-outs will have enough output to feed a signal to the Yamaha's line-level (amp-in) inputs.
Regards,
Alan
