That's one thing I haven't quite figured out yet on my GU-12. It's got some sort of weird setup for the output -- not your typical adjustable negative DC to the grids.
I'm fairly certain that it has something to do with that "E" feed from the center tap on the filament winding of the power transformer. But there's nothing adjustable from the factory, so I haven't messed with it at all. I did get the Bias-Rite, and I could probably check the cathode current through the two 7591s just to make sure they're reasonably close, and just leave it at that.
I forgot to mention re-capping. I've already done that on mine. Apparently the three capacitors in the tremolo circuit are prone to failing and shutting it down -- when I got mine, the tremolo was dead, but in my case it was the neon lamp / photoresistor module that was bad. So after that I figured I'd go though and completely re-cap it anyway; Orange Drops for the tubulars and new electrolytics.
I guess that if you're really a glutton for punishment, you can measure all of the resistors to see if any have significantly changed value from aging, too... but the GU-12 is working quite well at the moment, so I'm thinking that there isn't anything to far off. At some point I'm going to go through them all, just to make sure, though.
C.K.