I looked at the first page of
What causes resistor distortion?
Lets cut to the chase. I have over a thousand carbon comp resistors from 1961 to 1968 in dynaco amps and hammond & wulritzer organs. When I changed the 270 k & 470 k in my ST70 amp to metal film (vishay, multicomp/farnell), the idle hiss was substantially reduced. Maybe 3 db. This was predicted by this site. Under 100 k the effect is not so noticable. This is called "thermal noise".
In the distortion thread post about # 6 mentioned carbon comp value wandering due to moisture. Most posters on diyaudio.com are from Europe, and haven't seen carbon comp resistor 1961 to 1985? from Allen Bradley and Sprague. About 1961, some kind of paint was put on RCR07g etc. resistors that ran on the "mil spec" lines, even the ones with commercial part numbers that looked the same. These resistors do not vary in value due to moisture, unless overheating destroys the paint. Probably the US Navy was involved in the research. I've seen no announcements about the changeover. In particular my dynaco ST70 & PAS2 lived in 100% humidity Houston, TX the first 30 years of their lives and the carbon comp resistor values did not change: all were within 10% of nominal. Two lean post college years I didn't air condition my house, until mold grew on the walls.
There still might be a problem with value decrease of the 1-10 megohm ones; hammond repairers don't mention chassis build date usually. Hammond organs built in the Netherlands have been reported to have unreliable resistor values near the sea. Particularly an X66 owner that lived on one of the UK channel islands. He had to change every resistor out due to value variance; hundreds on this premium 200 kilogram model. Those resistors look just like the US made ones. The paint looks the same. Obviously, it is not. My 1966 to 1968 Hammond organs do not have carbon comp resistor value change, except the ones where underrated wattage burned the paint. David of NC, a guitar amp & organ repairman, says HH Scott equipment post 1961 also had the everlast carbon comp resistors.
As Hammond organs have a motor+tone generator whirr noise that covers up amplifier hiss, I'm not changing the carbon comp resistors: except the 10 k 1/2 watt in the power amp of two H100's, that were both burned and reading 11.5 to 12.5 K. Those were changed out during electrolytic cap replacement 2010-2012.