Well, to Osvaldo's point, I want to test/confirm that the bass pot (either the caps or the mechanical pot itself) is the cause of the issue. I did try disconnecting the two wires going from the tone controls to C3 and R15 for just one channel. This caused some other artifacts to appear on the waveform, but it did not cause the spikes shown in the video I posted to disappear. Does this rule out the pot? Or is there a better way to test whether it's the culprit?