Tony, There are a lot of tubes sold on ebay as NOS that aren’t There are a lot of lightly used tubes that still test above average new.
Without going too far into the weeds, tubes are usually tested for transconductance to determine how long they will last and to match them. Tube testers, Hickok is the most common brand, come with a scroll that tells you what the average transconductance is for a new 12AT7 tested on that machine.
Nobody has kept track of which tubes have been used and which haven’t. So if a tube tests above average new and the glass and pins look clean, many sellers will sell it as NOS. Others will sell it as "tests above new" but not call it NOS. There have been discussions about how to categorize old tubes that test above new but probably have some use as ANOS, i.e. almost new old stock., but that never got anywhere.
Tube dealers like Brent Jessee or Vintage Tube Services (Andy Bouwman) use more sophisticated testers than the Hickoks I mentioned above, although Hickok did make lab grade testers which are much more precise than the more common Hickok 600s or whatever.
Even with all the uncertainty, many people still feel it is worth it to buy vintage tubes because they had the same experience you did. They like the sound better. There is a LOT to know about vintage tubes, so you’re better off dealing with someone like Brent or Andy than trying to learn it all yourself.
