My guess? Power tubes need replacing and re-biasing once installed. New slo-blow fuses in all spots. If this isn't the case, it's potentially OTs which would be bad - real bad. Typically PTs go with a bang - pop, sizzle, crackle - this bit about there being a hum in advance of its death, not so sure. Don't like the sounds of this. When you're done going tilt on th gigging, plug this baby back in and hit the standby to RUN - make sure there's a load on it - and look behind to see if the fuses are shot. Even in advance of hitting RUN, when the amp is in standby mode, with the rear grille off the back, you should notice the tubes will either be heating and coloured homogenously, or there'll be a couple or four or maybe all six that look weird inside - red, orange, blackened interior of the tube, etc.
Peace,
Mo