Ref 2, MK 1 v. Mk, II

How much better?
Night and day difference. Much quieter, more musical, more tonally accurate, better control and no more mid-bass bloat.
I assume we are talking about ARC Ref 2 preamp. I've heard from many that the LS-25 Mk I can sound as good as the Mk II by upgrading to NOS tubes. And it's cheaper. I am sure this is also the case for Ref 2.
I had a request for feedback a while ago on this. I like the Ref II and the upgrade is fairly expensive and adds transistors!! You should listen to them in a side by side comparison.
Tubes can help, but I wonder about the basic redesign of the preamp, going to the so-called supertube. How super is it?
Do a search on the BAT preamps. I think BAT is one of the first company to jump on this tube. They probably have a lot of discussion on their website about this tube. And of course, compare to the Sovtek 6922/6DJ8s, they sound better. I suspect the redesign has to do with modifying the existing design to use this tube instead of the 6922s.