I would use the cables joining the modules together.
That way an output from one cable becomes an input to a module and eventually that comes out as a final output. That final output changes the supply conditions and that can give rise to a further change in the signal from the interfering cable you are testing.
Using a dummy load on a separate cable does not trigger the loop into a response.
You are right - all the wiring in these cases needs to be taken into account.
Certainly, they make nice receiving antennas.
I am amazed that there aren't serious hum issues with these amps.
However, I am concerned about the safety of testing.
So I'm going to copy the wiring practices I see in power amps.
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