SOmeone else can recommend screen resistor values.
The cathodes of the power tubes sit at a pretty steady DC voltage, with that bypass cap keeping it steady. That is a cheap and convenient place to steal a positive voltage to bias the tube heaters, instead of making another supply for the purpose. By elevating the heaters to a positive DC level, it prevents electrons from the heater flowing to the cathode. SUch AC curent flow can inject 60Hz hum into the preamp stages. If the heater is +15 or +25 or whatever, then the cathode will always be more negative, and will not attract those electrons.
It therefore reduces hum in the high gain stages.