Well, that amp is fortunately quite easy to signal trace. If the second double triode of bass channel is (like 6G6A is) a straight double triode cascade without any voltage dividing of audio signal between stages, it is very prone to oscillations if input and output wiring of that cascade is not kept clearly away from each other. Gain of that cascade is huge. So wiring is critical. If that stage has that 1Mohm neg feedback resistor, it is a bit tamed version. But still, if that amp was mine, I would add at least a grid stopper to the grid of the 4th triode of bass channel or a some kohms series resistor between triodes to form a voltage divider. Another very important thing of course is to make sure that none of tone caps and coupling caps are leaky. And make sure that those small measuring clips are really safe to use at high voltages.

I bought a 1969 Bandmaster with no transformers and substituted Dukane 1U460 transformers fired it up and there was a motorboat oscillation so I took all the pots, jacks, and removed the brass plate. It looked like someone used it for a spittoon. Or coffee. black tarnish, Scrubbed it with scotch bright turned it back to brass . Scrubbed the chassis too, wiped it down with alcohol. Clean behind the brass plate.

I had turned it on and it was “working” but it was 100% gain 100% of the time and the normal channel volume knob did nothing just like you experienced. The bass channel didn’t work at all.

The Eurotube meter is powered off the tube filament voltage.