Concerning 6SN7's, it seems to me that most of the noisy(kind of a spitty sound) and microphonic tubes I've encountered have been very old 6SN7GT's and their equivalents.That's a good observation. I'm not going to buy anymore tubes from the 1940s, and
maybe 6SN7'S from the 50's are a risk. Although, 1950s were the glory days for tubes.