Don't know that I would call that a driver tube, but if you like, go ahead. I just see it as a pair or triodes in series, like any tube amp. The signal hits the left grid, and the resulting plate signal is fed to the second grid. The second plate signal feeds a source follower to feed the power amp proper at SIGNAL A.
So it is low and farty. First check power. Looks like it runs on 70v rails, so are they both up close to that and clean? The op amps no doubt run on +/-15v or maybe 12. Are those all up to voltage and clean? The op amp ICs with 8 legs have outputs on pins 1 and 7, is there any DC offset on any of those? Looking for volts here, not a few millivolts. The tube supply is +375vDC, is that close to voltage and clean?
You know tube amps? OK, so are the voltages around the tube normal? With 375v B+, is the plate voltage on each triode a couple hundred volts more or less? Are the cathodes sitting at a few volts? Did you try a different tube?
Check out the signal path. It enters on page 2 through IC2A. From there through IC2B. All that stuff left of and below IC2B is basically a graphic EQ circuit that is not adjustable. They set it up for a tine and hard wired it.
The outputs on the 14 leg IC are the corners.
So apply a signal to the FX return and scope it through the amp. What does it look like at IC2A? IC2B? At grids and plates of the tube, and at SIGNAL A. That is page 2 and the lead-in to the power amp. If you don't have a clean signal at SIGNAL A, then you won't have one at the speaker.
Page 1 is the power amp. We'll look at that later.