Alright, I took some measurements with a multimeter, no power applied, as a base for continued troubleshooting. Here's what I found (tubes removed).
With the pot turned all the way CW, and the black lead to speaker black (ground):
1. The front center pin from the pot has a resistance of 103k
2. The back center pin from the pot has a resistance of 104k
3. These values correspond to the resistances measured from pin 2 of Tubes 1 and 3 (with tube 1 being the rightmost tube on the finished board.
With the pot turned all the way CCW:
1. Front center pin reads 7.8 ohms
2. Back center pin reads 8.3 ohms
3. These values correspond to pin 2 of tubes 1 and 3
With the pot at some random spot:
1. Front center reads 32.2k ohms
2. Back center reads 33.3k ohms
3. Correspond to pin 2 of tubes 1 and 3
At another random spot:
1. Front reads 84.5 k ohms
2. Back reads 85.7 k ohms
3. Correspond to pin 2 of tubes 1 and 3
There are three pins on the front of the pot and three out the back. The center pin apparently connects to the wiper, since it is the only one that varies as the pot is turned. The wiper position affects tubes 1 and 3, which are the pre-amp tubes? (from what I can deduce from the schematic
DIY Audio Projects Photo Gallery: Click image to close this window). The pot is 100k, so the max value is close enough. Also, it seems that the pot is fine, since the difference in resistances is not nearly enough to cause the issue I found.
Some more measurements determined that the resistances to ground match for all pins of tubes 1 and 3, and tubes 2 and 4 (as they should. When I say match, I mean well within 5% (which is fine, considering that the gold-banded resistors are only guaranteed to 5%).
I think I've eliminated the pot as the problem, and the tubes as well (since I've swapped them around and experienced no change). What should I do next? Voltage measurements with a live circuit? Should I be looking for bad caps?