I imagine you have both channels assigned to reverb and vibrato, right? It only has a 220K resistor connected at the PI input.
Measure resistors around this circuit: 82K / 100K / 2 x 1M/ 470R / 22K / 100R ...
An inactive triode in the 12AT7 it could also generate that problem. Measuring voltages on the plates could detect a severe imbalance.
If all is fine you could have a problem in the output transformer.