Hum hum hum

I have connected my NOS PS Audio gca 250 to my PS audio pre. Disconnected all inputs and I get a distinct hum at high levels on one speaker with just the amp/preamp/speaker connection. . Swich the xlr cables and the hum goes to the other cable. I probably will never play at these levels, but I would like to trace and fix the hum. Any ideas? Thanks.