First a WARNING: This amp and ones like it contain Voltages that are lethal and can cause injury or DEATH. If you do not have the training and experience working with dangerous Voltages, it is strongly suggested that you refer service to a qualified technician.
50Hz hum can only come from a couple of places, most hum in an amp is 100Hz. Check that each side of the 6.3VAC heater Voltage measures about 3.15VAC to ground. Next, check the two diodes on the rectifier socket with a DVM set to diode check with power off and caps discharged. If one is bad, this could cause 50Hz hum. Most 100Hz hum you find in the 6AN8 and KT88 circuits will cancel itself out in the Output Transformer. A matched pair of KT88 helps in this effort.
The Hot bias could cause some 100Hz hum. Check that the red wire from the output transformer connects to the power supply 'A' node. Some techs connect it to the Standby switch side of the choke like most Fender amps. This too can cause hum, but usually 100Hz. Check that you have high Voltage to pins 3 and 4 of the KT88s.
Looking a little closer at the Voltage readings you took, it would seem the Voltages around the 6AN8 are somewhat off, which is not unusual for one of these amps. The true test is to measure the power output of the amp into a dummy load. Bias to the KT88s looks like it is on the hot side, I would expect the bias Voltage (pin 5) to be more negative. Note that your meter may load down the Voltage if you measure right at pin 5. Better to measure at the bias adjust pot. Check that the plate structure (large metal pieces inside the tubes nearest the glass) of the KT88s are not glowing red (called red plating). This indicates the tubes are drawing too much current (a potential cause of 100Hz hum).
The first thing to check in the 6AN8 circuit is the resistance of the two 47K resistors connected to pins 1 and 3. These are carbon composition resistors which can drift in value with age. If they differ by more than about 5%, I would replace them. If the Voltage ACROSS one of them is off by say 20V, the Voltage ACROSS the other should be off by the same amount. Voltages to ground would be off, but in the opposite directions. Another problem which can effect that circuit is leakage of the two "Black Cat" capacitors that connect from the 47Ks to pin 5 of the KT88s. Check for Voltage ACROSS the two 100K resistors (should be near zero). You can take these measurements with the KT88s removed from the amp for safety. Use extreme caution and don't let the probe slip and short to ground.
6AN8s are getting hard to find, install a new or fresh one if you have it. If these and other components checkout, I would adjust the value of the 680 Ohm resistor (between 470 and 1K) connected to pin 9 to obtain the 80V on pin 3. Note the 75V indicated on the schematic for pin 9 is an error, Voltage should be more like 0.5V but depends on the tube. Adjusting the 680 Ohm should get the amp firing on all cylinders with full power output after a bias adjustment to the KT88s.
