When you pulled the top cover off the preamp did you notice the power transformer? I assume it is a toroidal. Correct?
Have you noticed the mechanical hum being louder or quieter during different times of the day?
Try the preamp late at night.... After midnight.
Any house sharing the same utility transformer as your house can put DC back on the mains.
I take it you live in a residential neighborhood, houses all around you? No businesses?
Have you thought of taking the preamp to someone else's house, not on your street, and plugging it in one of their outlets?
Cheap dimmers will cause DC on the mains.... But you said you turned everything off but the dedicated circuit.
If your problem is DC on the mains and it is not coming from your house you could buy a small isolation EI transformer. 120V in 120V out and plug the preamp into it.
.
Have you noticed the mechanical hum being louder or quieter during different times of the day?
Try the preamp late at night.... After midnight.
Any house sharing the same utility transformer as your house can put DC back on the mains.
I take it you live in a residential neighborhood, houses all around you? No businesses?
Have you thought of taking the preamp to someone else's house, not on your street, and plugging it in one of their outlets?
BTW, the Linn Classik is part of my bedroom system with some Tukans and is not on a dedicated line and I have quite a few dimmers in the house.The Linn might use an EI transformer instead of a torrid.
Cheap dimmers will cause DC on the mains.... But you said you turned everything off but the dedicated circuit.
If your problem is DC on the mains and it is not coming from your house you could buy a small isolation EI transformer. 120V in 120V out and plug the preamp into it.
.