I would not pull the big board unless I had a darn good reason. You think the problem on the power tube board? OK.
A bad screen resistor is a pretty sure sign the tube for that socket was bad. Or the tube was replaced after a bad one blew the resistor.
When you say you tested parts, may I assume you did it with your hand meter? A meter uses a volt, maybe two, for any tests, and does not put real world conditions on parts. A cap that doesn't leak at 2v might well leak like a screen door at 200v. The miniscule meter currents will not expose something that breaks down at 2 amps or even 200ma.
Same with transformers. Your meter will tell you if two windings are shorted together or to frame, but how would you know if two turns shorted together? You won't. Nor will a meter expose failed insulation in a transformer that arcs. You might tell if the primary of the output transformer was shorted end to end or in some other gross fashion, but that is about it. Arcs can only be detected in operation.
Go to RG Keens Geofex web site and there is a tech tip of a simple transformer tester you can make. It won't make it arc, but it will at least detect shorted turns. Really, justa battery and a neon bulb, it is very simple to make and use.
When HV fuses blow, we often suspect the flyback diodes, but if they are failing, it blows fuses without tubes too. So I don't suspect them today.
ANy tube that arcs inside - white flashes - is instantly on my bad tube list.
Take your four tubes and one at a time, sub them into some other amp, preferably one with similar B+ voltage. I won't assume they are good at this point.