What's the status of the amps themselves? Have the coupling caps (C2/C3) been replaced, or are they still original? I haven't experienced it myself, but having read others' stories, when they go, they may take out the valve and possibly the output transformers. It's worth replacing them as a matter of course.
Since the rectifier for the Q22 is in the amplifiers, it's worth having a look at the GZ32 to see if the voltages coming out of the amp back to the preamp are in spec. There should be about 330v on pin 3 of the six pin socket on the amp.
For the 22 preamp, C5/C6 and C10/C11 are 22µF electrolytics, so they should be suspected of being dried out and checked. You can also try to do the capacitors in the can, but it's really hard to disentangle everything.
Have you checked voltages
against the schematic?
You may have seen it, but in case you haven't the annotated images for the
underside and the
top side done by Keith Snook are invaluable for working on these.