What made you change to a 6SN7 preamp?

@bigkidz nailed the 6SN7/6922 comparison.

 Perhaps I shouldn't comment, because I've never owned a 6SN7 based pre.  

OK, as long as you insist.  

What I can say is that I've never been satisfied with my system without a 6SN7 tube in there somewhere.  There is just a magic that they impart to the midrange, especially with brass instruments, that has no parallel.  My old Modwright Sony CD player with the right 6SN7 tubes could do French Horns like no other source I ever heard. 

The popularity of this tube has inspired a lot of new production attention.  There are good-really good- new production 6SN7s out there.  They aren't cheap, but they are being produced and they measure up to and arguably exceed many of the top tier NOS tubes.  You can't say that about the 6922.  Good luck trying to find a NOS 6922 that will hold its own against those old Siemens tubes. 

My preamp uses 101D tubes, and I just love the sound of its Psvane WE replica tubes. That tube has similar virtues to a great 300B.  My source uses 6922s, and I sprung for the NOS Siemens tubes which are worth every penny. They are amazing tubes.  Sweet, smooth, great tone, fast and dynamic.  Superb.  I've carefully selected the 6SN7s in my Atma-Sphere M-60's,  I really think all of these tube designs adds something special, and none of them have glaring weaknesses. 

It takes good design/implementation, system synergy. and then finding the optimal tube choice for the equipment you have if you want to maximize performance. 

If I were to move on from my current preamp, I'd probably be looking for a 6SN7 based unit.  

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.