I have a non-plus 5:50 that was noisy from day one. Different tubes wouldn't lessen the noise. I lived with it. Just before my five year warranty was up I took the amp to the factory in Petaluma (short drive from SF) because it was sounding very inconsistent and more noisy. The servicing included fixing a tube socket I probably bent from trying so many tubes without pulling the chassis out to do the swap. I got home, and the amp's noise was mostly gone.
A few days later all of the noise returned, and the gain knob and some of the tone knobs were crackling loud.
So after doing some research I took the chassis out and used Deoxit D5 on all of the pots, tube sockets, and jacks. When I was done and tested the amp it was like I had a brand new, dead silent amp. It had never once sounded that noiseless. The cleans became amazing for the first time.
A couple weeks ago (just shy of a year since that Deoxit D5 treatment) a bit of the same static noise resurfaced, and some of the knobs had a crackling again. The same knobs were also stiffer to turn than the other knobs. I again took the chassis out and cleaned the pots, sockets, and jacks. And I got the same great result this time as well. Noise was eradicated. Since I'm not using Dexoit on one thing at a time and then testing the amp I can't say exactly where the oxidation was an issue. But both times there seemed to be a correlation between the constant, static noise and the crackling/stiffness with the gain and tone controls.
I also used Deoxit on the jacks of pedals that had been retired years ago after no longer working. All of the pedals worked again. I don't know if things are just more prone to oxidation in the SF climate, but I now start any troubleshooting by making sure the problem isn't simply oxidation that can be quickly cleaned.