[Techman]

It's possible that the lower reading that you got of 40K across R50 may be indicating some leakage in C49, the capacitor which it's across. You'd need to disconnect one end of it to isolate it for a definitive reading.

You'll get a low ohms reading across the mains input to the transformer as this is the DC reading, which is different from the impedance at AC.

It would be interesting to measure the actual AC voltage across the input to the transformer with the lamp in line, although as has been said, it's tempting to remove the lamp and apply full mains with caution. With regards to the hum with the lamp in line, this is a good point. However, it just depends on how much hum. I've just conducted an experiment here on a similar twin PX4 type amplifier on reduced voltage. At 150 volts input the amplifier worked well with clear music and no hum, but obviously with very much reduced output power. Between 180 and 200 volts some hum was evident. Between 200 and 240 volts the hum went back down to normal level of nearly none at all. I also conducted the test in reverse with the same results of the hum, such as it was, peaking between 200 and 180 volts to the input of the mains transformer - I was using a Variac for continuous voltage variation.

So the hum may be misleading depending on voltage input, or you may have leaky reservoir/smoothing capacitors. You may now need to apply full mains power to make definitive tests. It may be wise to check resistor R50 out of circuit. Please be careful when measuring mains voltage, even the other side of the lamp limiter, and also make sure you've always discharged those capacitors before connecting/disconnecting them, or from them.