The 348 could be defective, but not from anything you have shown us. Clearly the +15v rail is missing. Are the channel switching LEDs working? And the relay? They also run on +15. Is the 78L15 getting input voltage?
I see no involvement in bias from the +15v. The bias supply is just s simple supply.
So what bias supply voltage is on the grids? The zeners add up to 39v, do you have anything like that?
I am not sold on the adjust mod. After all, what it amounts to is a 150k resistance in parallel with R132. The pot will dial it down to 100k. So I think the range of adjustment is roughly 50k-60k.