Identify this hum, please

Ah. The hum was caused by noise pickup in your cables, and the impedance of the passive setup was so high it could not control it.

A tube preamp would of course be my first suggestion.

Its not the amps if the volume control makes a difference. Does it hum if the volume is all the way down?

It sounds to me as if you have a ground loop. This is likely caused by poor wiring practice in both the amps and the preamp working in tandem.

The wiring problem has to do with how the units are grounded through the power cable. Although Minorl is correct in his admonishment to not lift the safety grounds, I do understand the temptation to do so.

Here is my recommendation:

Go to the hardware store and get some of those ground cheater plugs. You are going to do a test to find out where the problem is.

Start by using the cheater on the preamp.

If hum goes away you have two solutions:
1) send the preamp back for repair to the ground
2) get an isolation transformer for it

Second, plug the preamp into the wall normally and put the cheater on the amps.

if hum goes away you have the same two solutions as listed above.

DO NOT use the cheater otherwise! It represents a shock/fire hazard should anything go wrong.

Alternative approach using a Digial VoltMeter (DVM):

Set the meter to ohms scale. Measure from the ground pin of the power cord to the ground of the RCAs.

If you read near zero ohms, follow the solutions above. If you read significantly higher than zero ohms (25 ohms or more), the unit is off the hook.