I would like to share my impressions from my first use of NOS tubes.
So far I have used 2x Sylvania 6v6 (black glass, 1950-60 years, about $100 for matched pair) and 2x Tungsram E80CC (1968, about $23 for matched pair)
Well, I have to say I'm very impressed. I was skeptical before that NOS tubes would be overly warm, muddy, inaccurate, and other such things.
Before that I used modern tubes from Genalex and it must be said that I bought a new set at the end of August, and six months later I got severe crackling and noise in one of the channels. Experimentally, I found out that the problem was in one of the 6v6 tubes. I bought this from a highly recommended store, and perhaps I have no complaints about the seller, as any tube can fail literally at any time.
With these NOS tubes I found complete silence, I can't even hear the slight hiss of the amp. Also to my surprise the sound presentation has excellent detail, very accurate, firm but deep, textured, thunderous bass, the soundstage is very spacious, which is important to me, it is also rich sounding, the sounds here have a wonderful body, volume. All in all, I can't pick on anything at all. For me these tubes completely beat modern tubes from Genalex for example.