Don't know if this is of use to anyone, but I've currently got an Ampeg Pre into an Ampeg power amp, rack-mounted on top of each other.
I was getting a lot of hum, but all of the cabling was solid, and the mains supplies were all on the same ring, valves replaced just in case they'd gone microphonic etc....
Turns out that having the amps so close to each other was enabling the two magnetic fields of the transformers to 'interfere' with each other to the point where my bass playing would come through the power amp even with the preamp turned down.
a sheet of aluminium placed between the two has eliminated all of the noise.
Simples.
Also, egg boxes make perfect holders for golf balls, except that they're slightly too small.