I've spent a little time thinking of a way that some type of measurement information from you would allow me to know if I'm recreating the about the same level of output as your 770 is experiencing, but haven’t thought of one using measuring devices that aren’t very good at audio frequencies. Perhaps an easy way to drive all channels equally at 60Hz so that the 770’s outputs can be measured accurately with common AC measuring tools. This would require setting all speakers to large or setting the crossover points low enough for all speakers that 60Hz is not routed away from the 7 speakers. If so, you could determine, for example, that at an output of 18 volts AC on all 7 channels, it takes 14 minutes for your 770 to shut down thermally.

My guess is that your speakers that measure about 3.8 ohms DC are probably about 6 ohms at low frequency AC, and the ones that measure about 5.7 ohms DC are probably about 8 ohms at low frequency AC. That would mean that, at 18 volts RMS AC, the 6-ohm speakers would have to sustain a continuous 54 watts and the 8-ohm speakers a continuous 40 watts without damage until the 770 shut down. I don’t know if that would be good for the drivers. Anyone with speaker driver experience want to comment on that idea before we attempt this?

I’ll be away from the computer for a few days, so I check back next week.