Hi,

Sometimes this can be caused by an open ground trace on the Power Amplifier. If the Ground Trace or the Ground Ring Connection to the Amplifier board is broken open, it can do this.

The return path is long (pronounced as "Ground Loop"), without that dedicated Ground and it gathers Chassis and other noise along the way and you here it.

I guess the same thing could happen in the Empeg too.
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In SI, a little termination and attention to layout goes a long way. In EMC, without SI, you'll spend 80% of the effort on the last 3dB.