"Sometimes an input wire from a turned-off source can be noisy...noise that goes away when the source is powered up..."
"I'm guessing that with the preamp off and thus an open circuit to the input, some noise is being picked up on the input causing the problem."
I bet it is something along these lines. I'll keep you posted if there are any further developements.
"I'm guessing that with the preamp off and thus an open circuit to the input, some noise is being picked up on the input causing the problem."
I bet it is something along these lines. I'll keep you posted if there are any further developements.