I suppose for a true apples-to-apples comparison, someone would need to mimic your setup exactly, including the exact music that you’re playing when the 770 overheats.

But for a kind of Red Delicious-to-Gala apple comparison, one might only need to have the same load impedance that your speakers provide driven by a 950/770 combo in the same mode and at similar electrical levels.

My speakers are 6-ohm nominal x 7, driven by a 950/770 combo. In what way could you measure the output level of your 770 that leads to overheating?