1945 RCA VT-231/6SN7GT with grey glass
This is one of my favorite tubes. It is simply grand and smooth, with good bass, an incredible midrange, and pretty good detail and highs. The tonality of this tube is just right. Vocals, piano, horns sound spot on, very musical. And then there is this huge spacious 3-D sound stage.
I have come to believe that the 6SN7 family is just as good as the 6DJ8 family of tubes, but different. The 6DJ8 is cooler with great detail. The 6SN7 is warmer with more musicality and a palpable 3-D soundstage. Both are excellent but again, they are different.
It appears to me that great detail and coolness go together. As the sound becomes warmer, the reverberation masks some of the lowest level details. But in exchange for the loss of some detail, the extra warmth adds more musicality and a spacious 3-D soundstage. So for the most part, the 6SN7 is not for detail freaks. But speaking for myself, musicality and a spacious soundstage are more desirable than ultimate detail.
Coming back to the RCA, I believe it strikes an exquisite balance between warmth and detail. And in my opinion, this belongs in the collection of everyone who wishes to explore the 6SN7 tubes. RCA manufactured this tube from 1942 through the mid 1950s, and the general consensus is that the 1940's production are the ones to get.
In conclusion, of the four 6SN7 -- CBS/Hytron, National Union, Tung-Sol "mouse ears" and the RCA -- I recommend the Tung Sol and the RCA as worthy additions to your collection. I have two more coming, another Tung Sol with round plates and black glass, and a Sylvania "Bad Boy". These have received very good reviews and I am looking forward to their arrival. :)