What made you change to a 6SN7 preamp?

This is of course, a good point, but I really think, based on my experience that the 6SN7 tube is just inherently a better tube.  Its inherent virtues are such that a talented equipment designer (AKA Ralph Karsten) can do some magic.  I was really surprised when Dan Wright moved from the 6SN7 to the 6922 tube in his digital mods.  I have owned both the Modwright Sony HAPZ1ES which uses the 6922, and the Modwright Sony 5400 ES which uses 6SN7, and there is no question that the HAPZ1ES is a better piece of equipment.  Stock, it was arguably as good as the modified 5400.  After modification, the HAPZ1ES was substantially better, but it took a lot of money to get the top tier tubes from the 6922 family to really allow the HAPZ1ES to reach its potential.  Ralphs observation above that the 6922 tube wasn't designed for audio is not a small point.   If you buy a piece of equipment using a 6922, you better snatch up a Siemens and a spare.  Even the Telefunkens and Mazda tubes aren't in the same class as those old Siemens.  Forget about new production. 

I really think a case could be made that the 101D tube is underutilized, if one judges strictly on the the tube per se.   That tube, properly implemented, has virtues that are hard to ignore.  It has two significant downsides.  It is vulnerable to microphonics, and there are zero NOS tubes out there. It is all new production, and the only great new production tube is the Psvane WE, which go for around $700 a pair. 

If I were designing a preamp, I would work around the 6SN7.  As a buyer, I chose the 101D based Coincident and have no regrets. except when it is time to spring for new tubes.  

I went the opposite direction.  I went from a 6SN7 based preamp to a 6922 based preamp.  I agree with many of the comments about the 6SN7 and happily rolled tubes in and out on a regular basis.  After spending many hundreds of dollars on tubes, I found 2 or 3 pairs that were my favorites, each of which had unique strengths, and enjoyed the ability to change the sound from time to time.

I was a bit apprehensive when I bought my Herron Audio VTSP-360 because I wasn't sure that things could get much better than my previous preamp, but I was immediately struck by how much quieter it was, how much detail it had, and how it presented as good or better of a soundstage and had very precise imaging.  Most of what I've read about it indicates that there are not major differences to be had by rolling tubes, and to date I have yet to open the case and am still happily using the stock tubes that came with it.

There's more to the preamp than just the tubes and Keith works magic with his gear.  I also own the Herron VTPH-2A phono stage which uses a combination of 12AX7 and 12AT7 tubes.  Again, magic happens with the stock tubes.

I was immediately struck by how much quieter it was, how much detail it had, and how it presented as good or better of a soundstage and had very precise imaging. 

It would be lovely if we could attribute the characteristics above to a particular tube; we could all just use that tube and then have state of the art. To address those characteristics has everything to do with operating points, bandwidth, parts quality, grounding, layout, power supply design and so on. The advantage of the 6DJ8/6922 is its linearity and bandwidth (it was used for instrumentation for example) so if you have a non-microphonic tube you'll do well. Since microphonics was not a concern in the applications for which this tube was designed (like television tuners) finding non-microphonic examples is a bit of a task.

Without naming designers, one shared with me he decided to use either 6DJ8 / 12Axx instead on his next version preamplifier because he felt the availability of good 6SN7 supply was dried up or hoarded away. That was 4 years ago.

Today we see more new production and updated 6SN7 / CV181 tubes showing back up from across the pond, more people testing them out, all hoping they last longer than a year or longer than any of their NOS stored away. I'm on that list myself. Not sure how this may or may not influence component designers moving forward. 

I continue to learn a lot from this thread. Thank you all so much.

@big_greg There's a Herron preamp listed used that I have my eye on....someone stop me before I ruin a perfectly good marriage.