I agree .. .call Mark at Rogue first. Then, perhaps, call Dennis Had at Cary Audio in NC. He knows a ton about that tube and uses it so often in his gear. He is a "23mA kind of guy" in his own words in that he likes that as the bias current value in his 6SN7's in many cases. It is not just the brand or age or production run or specific design of the tube, but also the current draw it has that also largely impacts the sound. More current draw typically means more rich, full bodied, and warm sound with greater coloration to those aspects or traits. It can be like listening to a tenor sax versus an alto sax. Lower current draw typically means thinner, more incisive, and less coloration. A 6SN7 can be quite coloring and warming on the sound as well as fleshing it out thickly so. Try several of different brands and current values unless you already know your preference for sound types in general as described above.