Krell Multiamp Chassis and Hum

I am connecting the Anthem 720 preamp outputs for Height1 and Height2 to a Krell multi-amp chassis.  H1 is connected to Amp1 channels 1 & 2 and H2 is connected to Amp2 channels 3 & 4.

This is a known working Krell amp but unfortunately there is a “VERY NOTICEABLE hum from the H1 & H2 speakers, which are very good KEF wall mount speakers, brand new.

The hum fades from high to low then high to low and just keeps going in this same manner for eternity unless the system is powered down.

I have installed ground-loop isolators/breakers, and line conditioners.  There is no change.  There is a powered subwoofer on a separate circuit with a ground-loop isolator/breaker

I have installed 12v trigger cables with ground-loop isolation.

If I disconnect the Anthem 720 pre-outs from the Krell, the hum goes away.

Next I am going to try connecting the Anthem pre-outs for H1 & H2 to another preamp (in this case a Krell Foundation processor) before feeding the connections to the Krell amp (the thinking here is that a ground loop may be carried over the preamp output connections).

Any ideas?  Suggested next steps?

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