Drew, as you describe the situation, the problem is almost certainly overheating alone. You didn't describe exactly how you measured the 100dB level, but assuming that your number is reasonably accurate for the max level, your M80s were using only around 20 watts for that peak. The 875 has been measured to be capable of more than 200 watts in such a situation. The protective circuits wouldn't have shut down because of excessive power/current being used.
As others have suggested, more ventilation room and/or a cooling fan would be advisable.
