It's the preamp. you changed 1 thing, problem started.
Preamp straight into the wall, no problem, thus the preamp is good, EXCEPT it is making .....?
I would get a length of wire for a new ground somewhere: start touching contacts, this one, that one: searching for some combo that ends the hum.
I did that on my new equalizer which hummed even in bypass mode, found connecting the left and right channel's grounds to each other solved the problem. Just hit or miss.