Since the 787 has a quite powerful amp section and the M80s are very efficient, it's surprising that you're having trouble. Of course there's no problem with connecting the pre-outs to an external amp, but I wonder if power is really the problem. Yes, the M80s are rated at 4 ohms and Onkyo says don't go under 6ohms, but that really shouldn't cause trouble. Possibly in two-channel do you have the volume so high that you're simply asking too much of the equipment(and your ears)?
A long-shot : the 787 has a ventilating fan to help prevent overheating. Listening closely, can you hear it running at times, or is it possibly not operating?
