5F1? They may or may not be, but I sure would not assume that.
What do you mean "start" with filte3r caps? Please don't just throw parts at the amp. It is broken, let's find the problem systematically and fix it.
Anything is possible, but I tend to doubt your filter caps all of a sudden just failed. When they do fail, we usually hear LOTS of hum, or fuses blow. They generally will not pull the volume down, and surely have no way to make controls no-functional.
Isolate the problem
Does it sound the same way in the headphones?
Your low voltage supplies steal voltage from the power tube cathode. SO check the cathode voltage at the power tube pin 8, says 27v, even close? And verify that pins 3 and 4 BOTH have B+ on them.
That cathode circuit is tapped half way down, and provides power to the reverb return JFET stage as well as the LDRs that enable the volume controls (channel switching).
Why would the cathode be void of voltage? COuld be a shot C19. Your B+ could be bad. If the screen voltage is missing - node C - then the tube won't conducts, and no voltage. B+ is a sort of doubler, so close to 500v on the plate and 250 on the screen. I guess that could be a cap, you may be right after all.