IME there are no bad 6SN7s, the hyped/expensive ones may be a tiny bit better in some gear but they aren't worth the price imo.... My favorite tube is probably the 5692, I have RCA and Sylvania and they are the exact same tube but the Sylvanias are right at or just under $100/pair and the RCA red base are closer to $150/pair. These tubes have a reinforced internal structure and are very quiet. Otherwise just about any Sylvania sounds very good to me, the Russian variants are fine but be careful as there are a lot of different ones. The 1578 with black hole-plates are the grail but nearly $200/pair. People try to sell any metal-base 6N8S for big $ though, only the one I mentioned is worth more than $100/pair. Some people think the newer coin-base tubes are not as good but ime they are very good, better than most early production 6SN7 and you can get them dirt cheap.
I rotate tubes but right now I have metal base 6N8S with black plates as voltage amps and coin-base no-name tubes as output tubes in my preamp....
