A hum which increases with increases in the volume control on the pre-amp is usually going to be found in a component ahead of the pre-amp, for example a tube (often a source, a failing tube) in a phono stage or out board unit, or a failing tube in a DAC/CDP. It could also be a tube in the pre-amp, but not so often when the volume of the hum is controlled by the VC because the pre-amp amplifing stage follows the volume control and the gain and hum component is constant.
It could also be a part (resistor, capacitor, etc) in a source which is going south. You friend needs to source the hum by going back to the basics. Disconnect everything from the pre-amp and see if the hum goes away. Assuming if it does, then add back one source at a time until the hum reappears. Once this occur he can address the issues possible in that source creating a hum.
Hope that helps a bit.
