I myself, do not own a hybrid amp, but I would go through it all. Check all diodes, output transistors, solder connections. Are those ELL80 tubes good? Are they glowing? How about the 12AX7 tubes?
I don't have the schematic so this is just off the top of my head, but someone else here owned the non-hybrid Fisher amplifier that looked like your amplifier, except it had the 7591 tubes, along with a couple of 12AX7 tubes, where the filiment voltage of the 12AX7 tubes were taken from the 7591 tubes cathodes.