I have been rolling multiple "NOS" 6SN7s in my Apex Teton but no matter what I use, be it my favourite Sylvania 6SN7W metal base, Tungsol BGRP 6SN7 or the Mullard CV181, I just found the soundstage a bit closed in, or collapsed as one might put it, and the sound not clean enough for my liking. In other words, there was not enough "air" - the sound was rather thick - too much so to my liking on certain types of music. I was already using the WE 417A which I found to be the most neutral and accurate of all power tubes I have tried on the Teton. For this reason, I preferred the EC 4-45 (running EML 45 tubes with the WE 417A) for classical and live instrumentals and started to question why others vouched so much for the Teton for its speed, attack and detail.
Then, my new CBS/Hytron 5692 brown base arrived. "Bam"! What a difference. The Teton sounded much more open, airy and dynamic, and I was getting pleasant results with classical and other well recorded live music that made the Teton compare very favourably to the 4-45 in this aspect. The key factor that distinguished this tube from the others was that the soundstage sounded "limitless" when it should be, instead of like a cavern on the HD800.
Now the question is - is what I am experiencing characteristic of the various 6SN7 tubes I have mentioned, or is it likely that all of the purportedly "NOS" premium 6SN7s I have, excluding the CBS/Hytron, are all past their prime or not as "NOS" as they were touted to be? Can there really be so much of a difference? This really baffles me because I have rolled 3 different Sylvania 6SN7W metal base tubes which I have purchased "NOS" and perhaps put on roughly 300 hours each: they all sounded pretty much the same to me. I wonder if anyone can enlighten me what is going on?