there are at least a few vintage tube dealers on the net with extensive documentation on their sites regarding the different versions of tubes - especially 6SN7s.
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A lot of audio equipment manufacturers utilize the 6SN7 and millions of these tubes were manufactured for the military, so there is a good supply and more choices for tube rolling. My favorite 6SN7s are Slyvania for accuracy
For tubey sound I like RCA and Tungsol for smoothness
Agon Member JSAUTTER is a very knowledgeable source for tubes
Brent Jesse is a good dealer on the web. I have no comment or opinion on the 6np1
I have Cary SLP 98.
I tried few 6sn7 tubes in the front and always have sylvania gtb at the back.I tried tungsol brown base,sylvania brown base and few more and sound really good..Then I tried these 6sn7 tubes from russsia with a 1578 stamp on the glass ...then I stop looking for more 6sn7 and always happy listening to music now.I recommend these tubes to all A'gon member who own a Cary Slp-98 pre-amp and what a difference.Please try it!!Don't ask me now how it sound but i do believed you will be happy with this tube especially mate with cary slp-98.
the russian 1578 tubes, if genuine, are exceptional. i use them for input and sylvania bad boys for output on my wyetech opal pre, after much experimenting, and it is a great combo. ken-rads add more warmth if you like that. 6sn7's are widely available and offer many tube-rolling opportunities to get the sound you want.