Restoration/mod of mid-1960s PP 2xEl844 4x6AT6 guitar amp, voltage, hum problems. Similar to Fender 5F10 schematic
If your hum problem is 50 or 100 Hz, my first step should be to remove V4 (of course no power) and check. If no hum, check removing V3 and so on V2 and V1 to find where this noise comes from.
Check heater wires are twisted and as far away as possible from signals, specially in pre-amp.