I don't think that resistor by itself caused the hum.
It could have been done earlier with a bad tube and damaged the resistor changing it's resistance.
I don't know how hot this resistor gets maybe it should be a higher wattage so it don't cook itself to death.
What would be good is if some other people took pictures of this resistor and I could see if they are cooking.
Yeah that will be great if others can post some close-up pics of this resistor on their CS-300.