Got a reply email from Mojotone and sent them the pics of the loop installed, along with close up of the discolored 10k resistor.
They had asked about where I was connecting the B+ for the power supply.
Waiting to hear what they say.
Brought the amp upstairs and tried it out on the Alnico cabs.
Definitely something wrong going on with the inputs.
Low input is too quiet, and the MV seems to do very little.
Signal comes through with the MV on zero, and doesn't get much louder as the MV is turned up.
High input goes from off to super loud at 1 on the MV.
It's going back down to the bench and I'm going to disconnect and rewire both inputs and the pre-amp vol pot.
I had visually verified the values on all the associated input resistors and caps, but If I disconnect everything, I can run the individual components through my little magic tester gizmo to check if they are actually what they should be.
Probably wouldn't hurt to do another once over on the rest of the amp while I'm at it.