I didn't see the typo...
I would love to speculate on the cause of that hum (seriously). My first bet would be a contact problem in the ground line of one of the pre-amp inputs, which was so subtle that it went away when the temperature of the amp case increased to a certain point. Perhaps, it went away permanently because, when you "threatened" the amp, you might have also re-seated the problematic connection somehow... Can you swear that you did not change or tweak the setup when the hum "spontaneously" stopped?
