I tried this. It doesn't seem to do much. The only thing I did differently with this project is that I built a custom case – albeit from the original drawings, so it shouldn't be different in any major way.The cure was to screen the tube. I used some quite expensive gold plated bronze shields (it is best to prevent using iron/steel for this). But if your tubes are sensitive if the hands gets close then it could pickup other kind if noise also and then a shield could be a solution. You can see the shields I use in my avatar. My first test was to use a roll of solder to put over the tubes and that helped a lot. Maybe you can try something similar or maybe alu-foil as a test?
I'm noticing now that I do not have the washer and nut that connects the pot to the chassis. I had the front plate custom machined and iirc didn't bore out the pot recess deep enough to capture that nut.What is important also is that the pot has galvanic contact with the chassis. I spent some time removing the black anodizing.