My Line 2 came back from up-grade to the SE level with NOS Brimars, and they were too soft and uninvolving for my taste. So I put a pair of Brimars in just the 2 driver positions (gain positions), and used stock Sovteks in the other four positions. This was better than 6 NOS Brimars.
But recently, a friend let me audition a set of six NOS Sovteks (1970s), and I liked them so much I bought them. They were un-used and from Upscale Audio. The NOS Sovteks are distinctly more liquid with more "air", detail, smoothness, and are more holographic than the stock Sovteks-- and also much more dynamic than the Brimars. I've tried other NOS tubes in the gain positons and did not like the (usually) excessive bloom in my system.
I do think it's a system specific thing, and of course personal preferance, eg I like the stock Sovteks much better than the NOS Brimars-- they are much more dynamic, although a little bit dry, IMO. Good Luck. Craig