Here is the deal: the current that goes through the resistor is strong (on peaks) that resistor can't deal with it. It was working more than 40 years, so may be it's time to replace it with a better one. Every part has his life span, even the chassis
There are resistors that even though they can handle the power, but cannot handle the voltage.
Example: Ohmite 50W resistor at 1000v will dissipate 20W of power, which is fine according to the ohms law, but it's rated only at 600V, so he will heat up very quickly, to more than 200Deg. According to the de-rating graph from the same datasheet it can't be used at full 50W, so check the voltage also when you will be buying new resistors
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5W %5 cement wire wound is good enough for that application.
More closer screens to the B+, the more current they will draw. Problem of every GUITAR amp.