If the hum is coming from the amplifier itself, it is either a rectifier tube chattering or DC on the AC line or both.  If there is DC on the AC line it will make large power transformers like those used on the amplifier, vibrate.  If it's just a tube, change the tube and see if the problem goes away.  If not consider getting a DC blocker from Decware or Van Alstine or other reputable audio company.  

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