Dirty footswitch jack was making intermittent contact, briefly powering the channel switching vactrols on and off which made the amp sound as if it was fading in and out.
I also modified the tone stack for a more "British" flavor, and added a 3-way bright switch and midrange control. I rebiased the phase inverter for less gain but reduced the pre-PI attenuation in the clean channel. Now both the output stage and PI start clipping with roughly the same input volume, creating a more dynamic and musical distortion in the clean channel.