Well, you have narrowed down the problem to not being the tubes. Might be the power supply, might be a ground issue, might be an issue with the heater bypass cap if it is floated, might just be the way the preamp was made. Could be that a recap of the power supply filter would solve the problem. Hard to say without having the amp on the bench. Roger Hug was working on George's stuff a few years ago, but unfortunately Roger passed away in 2018. Don't know if PB might be willing and able to fit it in, you would have to ask him.