What made you change to a 6SN7 preamp?

@lowrider57  Thank you for relaying the story about that upgrade. It bears on my question directly. Very helpful.

@moofoo  Your example, too, bears on the question. And as others have said, there's no substitute metric for actually hearing it. Expense, tube, design are all relevant to the question, though. 

@big_greg  Thanks for your comment about the Herron. I need to sell something before buying anything else, but I'll keep my eye on that Herron. My Quicksilver got upgraded with Mundorf and Vcap caps and sounds pretty good. A night and day difference would really be the reason to change.

There are lots of possible tube to use. My favourite preamp uses 6GK5s.

I think you were reinforcing the point I was trying to make that it's not all about the tube? 

That is true. But if you work with tubes over time, you get a feel for what they do. More to the point though that in order to really compare what the tubes do, you have to make sure all the other variables (circuit topology, part quality, grounding technique, power supply design and so on) are put to bed.

We see this going on in the SET world all the time- SET fans say they've compared SET to Push-pull, but when you dig into it you find out that the variables I mentioned above were uncontrolled in the comparison. This makes comparisons invalid; all you can conclude at that point is you might prefer one over the other without knowing why that is if you're being truthful with yourself.

Unrelated to the thread topic but important to those looking to optimize their purchases regarding NOS tubes, Andy @ Vintage Tube Services is the guy. Give him a call, you'll be rest assured he knows his tubes and upon ordering you'll also find yourself grinning from ear to ear. He's a passionate and perfectionist audiophile.

@atmasphere 

when you dig into it you find out that the variables I mentioned above were uncontrolled in the comparison. 

Your understanding of these variables, and your cautious way of taking variables into account when you make comments, make you an invaluable voice in this forum. Thank you.

Lack of control over variables is what leads discussions into sometime wild and fun or confusing and conflictual directions. Of course, the room is almost never mentioned as a variable in almost any discussion, but that of course is a hard one!