McIntosh MC451 mid-hum (tube fed). No hum on tweeter, or on solid-state side. Anyone?

Thanks @erik_squires 

If you have no inputs at all connected but you have hum on an output, especially since it’s just 1 section, it is probably a bad power supply.  This can be a leaky rectifier or cap.

On the other hand, you should also check to make sure your AC circuit has no other devices running and that the neutral to ground voltage is 2V or less when your amp is on.

Get an AC multimeter.  Disconnect all of the inputs.   Set the meter to AC and measure the output on both, no need to use your speakers. 

Multimeters are really good in the 60-120Hz range so you should get a good sense of how much hum you are getting. You should see nearly zero, but 1-2 millivolts (0.001 V) is OK.

@erik_squires  this amp has 2 sides - 150W tube, 300W solid state. That is why I am trying to figure out with someone who has one. 

If they have the same thing. 

I have done a bunch of power swaps with cords, different outlets, conditioner - no conditioner.

I really don't understand what you mean by tube fed tweeter and mids.

Power supply hum, ground loops or otherwise, won't be heard by anything with a high pass crossover.  They are at 60 or 120 Hz and easily filtered out by a tweeter's high pass filter at 2 kHz or higher.

If you have a hum without any input cords it's likely to be a power supply issue.