It's a preamp tube
here the lis of what each preamp tube affects
I would remove one by one and inspect them.
V1A-- Input Stage
V1B-- 2nd Gain stage CH1/CH3,
V4A-- Reverb SendV4B-- 2nd Drive stage CH3
90 Watts = V8, V9, V10, V11 45 Watts = V8, V9
10 Watts = V8, V10
3rd Gain Stage CH2
V2A-- 2nd...