Interesting that I mainly use the 6SN7 tube family in my preamp and have not experienced any noise or microphony issues with older tubes. Just a couple outright failures.
My preferred 6F8G tubes are Raytheon with flat ladder plates (very hard to find). Step up from the T plate version. National Union round plates and RCA Gray Glass are also excellent.
6SN7GT VT231 TUBES
I love the old VT231 tubes!
The best sounding 6SN7GT ever...Sylvania, Raytheon, RCA, Ken Rad Etc. They may still be found, if you have $$$!! Where can I find true NOS VT231s? Any favorites that I missed?
(TS never made a VT231 - but their old black glass tubes with oval micas sound great!)