In case you are forgetting, the M80 is 25+ years old so maybe the heat build up could be because of components are no longer operating within their specified tolerances. Its just a guess if its this model...
As PENG said, he's trying to pull air from front to back the amp instead of blowing air across the chassis. If improperly setup which seems to be the case, the pulling of the air is not being effective.
I have 5" clearance on top with no fans and its running cool.