Go buy a can of tuna and make yourself a tuna sandwich.

 

First, any copper wrapping on the toroid will act like a short circuited turn and may lead to a lot of grief. I am suggesting measuring the resistance of the different windings in each toroid and comparing them. A single short circuited turn will cause problems on occasion.
I think that I read that moving the toroid changes the hum, and so it may be that there is an unintentional loop in the one package that is not present in the hum-free one. That would be a reasonable cause, but a big challenge to locate.
One other thing to try is swapping the two power transformers.

I love to try swapping them however they are hard wired into the power sockets and to the on off switch and potted in with black glue like compound and it's almost impossible to get off without taking a lot of time, I had to do it with the main boards on the problem unit, It's for Keeping sound inside the boxes.

If the problem returns I'll try it but for now I've got the hum under control and sounding good.

 

If you can find the conductor that is creating the hum pick-up loop and rout the conductor differently you can eliminate the hum.