Well, it looks like I wrote my last message a few days too soon. After over 2 weeks of static-free operation, my Two-stroke started up again with the static noise again yesterday.
At one point, the static became so pronounced and sustained, that it continued even after I turned the volume knob to zero (this is not typically the case – usually the noise goes away when I dampen the strings or turn the volume down).
Why would the noise go away after Robin re-tensioned the sleeves in the power tube socket, and then return in a few weeks? And then repeat this pattern by vanishing after I replaced the tube sockets, but then returning again in another 2-3 weeks?
Any ideas about what kind of problem would manifest in that way?