Studio Preamp Issue: Master and Lead don't work, low volume

Hello, I've spent a lot of time lurking and searching, and now am registering to see if I can get any help with my Mesa Studio Preamp.

TL;DR: My Master, and Lead Master controls on my Mesa Studio Preamp don't function, they have no affect on the sound or volume and leave me with very low output and thin sound. The earlier gain stages work, and the effects return works. I've tried some different tubes and tested the Pots with a multimeter to confirm they do something when turned. Any ideas?

I recently bought a Mesa Studio preamp (made in '89 according to the circuit board) from eBay. It worked well for the past two months into a Carvin TS100 and a Carvin Legacy 212 cabinet. Then last week, it randomly started acting up, getting very quiet and thin sounding. The Master, and Lead Master, have no audible effect on the sound. They do not affect volume, output, or tone in any way. Literally nothing, in either clean or Lead.

Schematic here for reference: http://tubefreak.com/studio.gif

Things I have tried

I confirmed it wasn't the Carvin TS100 by plugging a Peavey Rockmaster(with it's own issues) and a Line6 POD X3 Live. The line6 confirmed that the poweramp and cab are functioning properly.

The tubes in it are quite old, the person I bought it from said it's been in storage for around a decade. I tried swapping the tubes to another old set of 12ax7s I had laying around. They were questionable, with a couple microphonic sovtecs, but didn't seem to make a noticeable difference in sound. I also tested each of them in the V4 slot (left and return Effects return into Output A and B). By plugging into the effects return, and testing the output from the preamp, I got a loud, clean sound to the power amp from each tube, on each side. Therefore, at least in isolation, both triodes in each tube is functioning cleanly and clearly.

I have gain on the lead channel, affected both by Lead Drive, Volume, and Treble, so the early gain stages seem to be working properly. Because the effects return and EQ work, I have reason to believe the late gain stages are working.

I tested the LDRs with a Multimeter while the amp was unpowered, and they all held a resistance in the 10k-100k range. The LEDs glow brightly and appropriately.

I have confirmed the filter caps are not shorted.

I have used a multimeter to test the Pots, and the Lead Master reacts as expected, ranging from a high resistance to 0 resistance. The Master behaves more weirdly, ranging from a low resistance, to a higher resistance around 5, to a low resistance again. However, there is a resistor bridged across the two outside terminals so that is probably affecting the sweep?

I have tried chopsticking around in the amp to search for any wonky solder joints, but didn't find anything yet.

I have not tried.

I have no touched the 12AT7 that runs the Reverb circuit. Now that I look, that is located directly after the master and lead master in the circuit, so pulling it or trying a 12AX7 just to troubleshoot should be first on my list when I get home tonight.
Edit: I've tried both a 12ax7 and a new mesa 12at7 I just picked up from the local shop. No change.

I have not confirmed that the caps are fully working, just that I could see a charge held with a multimeter, and that they are not shorted.

I haven't been able to check exact values of resistors because my multimeter is questionable, and everything I tested was still in the circuit and subject to residual capacitor charges.

Anyone have any ideas? I'd love to get this back up and running.

I tracked down the issue, and it makes sense in hindsight. I'm hoping this double post is alright in the name of completeness (In case this thread ends up in google or board searches).

R213, the Plate resistor above V2a had a cracked solder joint. It just took a while to see because it was hidden underneath a big orange Capacitor. Makes sense because it is the only link between the B+ voltage and the signal input to the Master and Lead Master knobs. Tracked it down by measuring the plate voltage on each of the 12ax7 triodes (pin 1 and 6) until I found that V2's first pin was at a slight negative voltage instead of the 280ish of all the other pins. Then when I knew I wanted to find THAT plate resistor I could really track it down.

Confirmed it for a glorious moment when I has the resistor pushed with a chopstick and confirmed the knobs worked. Tried to resolder, and now whenever I read the resistance of that resistor I get >100 MOhm(or off the scale), instead of the 100 kOhm it should be. Think I probably fried the resistor while resoldering it, time to order some resistors I guess.