If the cheater plug works on the preamp then an isolating power transformer will break the ground loop (1:1 ratio). I suggest you use it on the preamp so it does not need a lot of power output.
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I think you have a ground loop issue. Turn off all the equipment.
PLEASE REMEMBER TO TURN OFF THE POWER AMPS EVERY TIME YOU CONNECT/DISCONNECT THE INPUTS TO THE AMP.
Disconnect the inputs to the amp at the amp side and power up the units. If the buzz is gone then you are creating a loop with the interconnects and power ground. Try using a cheater plug on one of the units. If the problem goes away leave it like that. All the units will be connected to each other via the interconnect grounds and will have ONE connection to the power ground when you reconnect the interconnects.