The REF10 is capacitor coupled, it should not be a DC problem.
Probably a faulty tube that starts oscillating at ultrasonic frequencies.
The best way to diagnose is connecting an oscilloscope to the output of the amplifier and slightly tapping the tubes with a pencil (including power supply) - sometimes it triggers the oscillation.
You can also swap all the tubes between channels and see if the problem moves to the other channel.
It can also be due to RF pickup - do you use un-screened cables?
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