Top resistors

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.

Many thanks Kirkus. I appreciate you taking the time to look at the schematic and offer such a thoughtful reply.

I have PRP's in the positions you mention. In your opinion, would it be worth swapping in low noise resistors or will the benefit be marginal? It's probably hard to know the results for sure, but thought I'd ask anyway. I'm willing to upgrade if it offers an improvement, but don't want to throw money at the situation when it doesn't do anything.

my opinion changing resistor doesn't make significant difference such we change tube or capacitor ,although they still have difference sonic character ,another good resistor that i recommend is Riken RMG ,i was ever been compare with vishay S102,the riken have better midrange reproduction & warmer sound than 102 ,but 102 have better tranparancy & clarity.i have never been compare to TX2575 yet

Mr Y- Any components(passive or active) that can be replaced in the signal path(or power supply) to provide a more transparent conduit (and gain) for the signal will have a cumulative effect. Many of us desire to provide as transparent a path, from the source to the speakers, for our music as possible. That entails replacing anything that adds or subtracts from the original signal. Some components(generally utilized in manufacture for the sake of economy) are known to induce noise, warmth or other colorations, and are targeted for elimination. What may seem insignificant or desirable to some, is found unacceptable to others.

yes i agree ,but warmer sound in riken rmg doesn't mean much of coloration like allen bradley ,but i mean they didn't sound too cold like S102 ,ussually more transparency ,more clean ,make sound become cold ,the sound like new mark levinson or 6H30 tube ,but this depend on personal taste.