Check with Frank Gow at Audioclassics.com - he may be able to help or steer you in the right direction - they sell and refurbish Mcintosh equipment and have done so for years.
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Are the Mac's on a dedicated circuit? If not you may want to look at whatever else is plugged into that circuit. Some dimmers and motorized devices sometimes can be culprits. Especially suspect would be anything with a high/low switch in it. I had a room humidifier with said high/low switch and it made my velodyne subwoofer amp hum in the low setting.
Try unplugging suspect devices to see if you can locate a source for the hum.
If you've narrowed it down to a transformer hum, try to find which circuit the amplifier is running off of. Then check the circuit breakers in the service panel.
Chances are that your amps' circuit is on the opposing phase of some of your other audio circuits and therefore generating some AC noise thats affecting the transformer. Have an electrician move all of your audio circuit breakers in the service panel to the same phase of 110 volts.
I moved all dedicate audio circuits to the same phase and that cured my transforer hum.