Are the power amps still working.
A buzz/hum that is not a groundloop, or a mechanical transformer buzz must come from the power supply.
Maybe a faulty smoothing cap in the supply.
Especially when it started slowly.
Measure rail voltages with a DMM set to AC.
If you intend to go 'playing' in there take great care as the DC supplies are around 80 Volts or more (160 Volt total) and the capacitors will make a heck of a spark if you accidentally short something.
Unless you can identify the 'smell' it may well be beyond mere capacitors.