This is a bit off topic because it relates more to 6SL7's than 6SN7's, but I wanted to share this. A few months back I built a SP9 octal, and the customer wanted NOS tubes in it. I found a seller who had a full case of surplus WW2 JAN 6SL7/VT299's. I ended up purchasing 12 of them, and used 4 them in the SP9. I have built multiple noval SP9's using various new and NOS tubes, also multiple SP9 octals using various new and NOS tubes, but as soon as I listened to his preamp in my system I was immediately struck by a whole new level of detail, imaging and life these tubes brought to the music. I ended up listening to it all night! Simply BY FAR the best sounding SP9 I have built, and one of the best preamps I have heard, bar none.
Next day I bought 12 more for a total of 24. I still have 8 of them, they aren't for sale at any price! 4 are in my preamp, 4 for spares. The new production 6SL7's I have tried don't even remotely compare to these tubes, the only other tube I was even half as impressed with in an SP9 was 1960's GE long plate 12AX7's in a standard SP9 which uses noval tubes. So I guess my point is tubes can make a HUGE difference in sound quality. I have two circuit boards for my SP9, one that uses 6SL7's, one that I modified to use 6SN7's. My favorite 6SN7 so far is the Sylvania "Bad Boy" . I was fortunate to find a quantity of them also. But having read many "glowing" reviews of the legendary Tung-Sol round plate 6SN7, I'm going to try the next best thing IMHO, NOS WW2 JAN round plate National Union 6F8G's. About 1/10 the price of the Tung-Sols, but reportedly sound as good. They do require adapters. Fingers Crossed, cant wait until they get here!!! If they are as big a leap as the JAN 6SL7's were, I'm in for a treat! Here is what I thought was some very good info on 6SN7's, and tubes in general: http://www.hifi-tunes.com/the-evolution-of-a-constant-glow-in-high-fidelity/
My experience parallels what they are reporting in the article.