If the problem happens with nothing physically attached to the inputs, and you’ve tried swapping tubes I’d say you have an amplifier problem and should have it serviced.
It is possible that for some reason you have one channel slightly more susceptible to RF than the other channel, and it’s a design issue.
Despite my suspicions it is worth attempting to identify if you have a particularly troublesome source of noise in your home. You can help identify EMI/RFI sources by turning off every other circuit except your amplifier. Also turn off any lights which are left on, and unplug network devices still powered on after shutting down the rest of your home.
I’ve heard of all sort of crazy things, from Wifi routers, LED light strips and home automation devices being the causes of interference so that’s why the breaker method is nearly fool-proof unless you forget to turn off lights and or have a wall wart plugged in and energized somewhere ont he same circuit.