Noise from resistors is based on the amount of current flowing through them, and there's virtually none flowing through R18, R19, R20, and R21, especially at audio frequencies. These are "grid-stoppers", whose purpose is to create a high-frequency pole in the amplifier's open-loop response, in conjunction with the output tubes' grid (Miller) capacitance. Don't worry about these.
If your want to swap some resistors, I'd start with R2 - the voltage that is amplified by every other stage begins as a current through this resistor. Second would be R12, then R3 and R4, as this makes up the feedback ladder. If you change R12, you might put a high-quality polystyrene cap for C4.
For plate-load resistors like R2, you should keep an eye on resistor voltage ratings as well. IIRC, the Vishay S102 is rated at 300V, so if the 'naked' derivitives are rated the same, you should be fine here.
If your want to swap some resistors, I'd start with R2 - the voltage that is amplified by every other stage begins as a current through this resistor. Second would be R12, then R3 and R4, as this makes up the feedback ladder. If you change R12, you might put a high-quality polystyrene cap for C4.
For plate-load resistors like R2, you should keep an eye on resistor voltage ratings as well. IIRC, the Vishay S102 is rated at 300V, so if the 'naked' derivitives are rated the same, you should be fine here.