I took some brave pills and popped the lid off this evening. (The catches on those Switch Craft plugs had be vexed for a few minutes! But I worked it out eventually).
I had a tap around the board with an eraser pencil, and for sure, there’s microphonics on both channels like you say. But CH2 is much louder.
I’m fairly confident the microphonics are from the transformers, it’s loudest if I tap on them or the immediate vicinity.
But like you say, I can live with microphonics to a certain extent, it’s the 50 Hz hum which is the real concern.
I did notice that the AC for both lamps runs right long the top of CH2 input circuit and is tied to the various wires coming from that area, so I wonder if CH2 is simply amplifying AC from there. I guess a few minutes with a soldering iron could test that theory.
Finally, the only thing visually amiss are the four output caps, they are all slightly domed, so it could be they need replacing. But conversely, the input caps look just fine.
I did try the sum and difference outputs too. I can hear the hum on both, although it’s far quieter. Given all the output caps look suspect, it possibly proves nothing.
At least channel 1 is good, so I can crack on with the vocal session booked for this weekend 
P.S. This reminds me of another similar issue on another bit of kit….I’ll start a new thread 