Hum hum hum

What exactly do you mean by switching cables, and hum goes to the other one?

Does this mean the hum is going from one side to the other? If so, bad cables. :)

E

When I switch xlr cables, the hum goes to the other speaker. I bought new xlr cables and same thing happens. 

Disconnected all inputs and I get a distinct hum at high levels on one speaker with just the amp/preamp/speaker connection....  I probably will never play at these levels

I think you are saying that the hum occurs at high settings of the volume control, and varies with the setting of the volume control.  Correct?

When I switch xlr cables, the hum goes to the other speaker.

When you say "switch xlr cables" do you mean that both ends of each cable are moved from one channel to the other?  Or that just one end of the two cables is switched between channels?

Disconnected all inputs and I get a distinct hum at high levels on one speaker with just the amp/preamp/speaker connection.

Does the hum still occur when a powered up source component is connected to the preamp, and that component is selected by the preamp's input select switch?

Also, assuming that the preamp and amp have 3-prong power plugs it may be informative to temporarily try a cheater plug (a 3-prong to 2-prong adapter, with the safety ground left unconnected) on the power plug of one of those components.

Regards,
-- Al