Hi,
I'm familiar with that sound also. Although I've never had this happen to any of my systems I have isolated this phenomenon twice for other people. One time it was a switch and the other time it was a potentiometer. Just a couple of ideas worth what you paid for them. Also, have you tried switching out the speaker cables? Look closely at the soldering of the terminations. Final thought: have you cleaned the chassis sockets for the tubes? Good luck.
I'm familiar with that sound also. Although I've never had this happen to any of my systems I have isolated this phenomenon twice for other people. One time it was a switch and the other time it was a potentiometer. Just a couple of ideas worth what you paid for them. Also, have you tried switching out the speaker cables? Look closely at the soldering of the terminations. Final thought: have you cleaned the chassis sockets for the tubes? Good luck.