Thanks to everyone for posting. Just a couple of thoughts on what I've read so far.
First, clearly, people hear and can articulate a difference; I see patterns in the kinds of differences heard. This is helpful to me in anticipating *why* I might want to try a 6SN7 based preamp.
Second, @decooney may throw some of the claims into some doubt with his statement:
another factor in responses so far may also relate to some people jumping up a level or two on design and improved parts quality with their next tube preamp purchase.
Strictly speaking, i.e bumping from something like a $1500 12Axx preamp up to a $3000+ 6SN7 based [a higher quality] tube preamp as a normal course of order.
Why? Because if some of you *simultaneously* purchased a better preamp AND a 6sN7 preamp, you could be mistakenly attributing the improvements in sound to the tube rather than the overall construction.
Thus, a question which controlled for this would ask whether your change in preamp was lateral in terms of construction quality and nevertheless there were improvements in sound which could safely be attributed to the tube?
I realize this is a very hard question to answer, for many reasons. But it is effectively the challenge implicit in decooney's observation.