I spent hours looking for every other cause, and even swapped out the choke for a new one.
I re-flowed all the ground solder joints and ... ug hours.
In desperation I ordered a NOS 6u8 and a 6u8 -> 7199 adaptor, and the amp is dead silent with it. I tried both channels with the new tube and each is silent with it, and hums like crazy with one of the old tubes.
This should be 60hz hum no? I still can't explain why I'm getting 120hz from the microphonic tubes, but this was definitely the problem and fix.
Note that the PAM's being plugged in must tax the power supply more, and cause slightly more aggressive power supply vibration shakes and that is why the hum got louder with them plugged in. The tip off here was a) trying everything else first, and then b) feeling the chassis vibrate more with a PAM plugged in.
I'm both thrilled and mad at the same time .