Michael,
My 5.1 has had the line stage upgraded, all rca jacks replaced, one set of tape outputs was converted to another set of main outs, the phono stage was upgraded, and the mc transformers were installed. Quite a lot of upgrades.
As for the SA-2, I am only having it repaired. It sounds GREAT as is. The reason Mike doesn't list any upgrades for the SA-2 is that they are very rare. If they were being made today, the price would probably be $3K. He has the comment "contact me for upgrade info" listed under the description of the SA-2. Knowing what I know, if I were you, I would keep my eyes out for a SA-2. I would consider having your phono stage upgraded, but I wouldn't add those transformers on a bet. If you want transformers, I will give you a good deal on mine when I have Mike remove them. I have the latest and greatest Sowter 8055 transformers and they were just purchased and installed in March.
As for the 5.1, most people don't realize how special the power supply is and how great the preamp really is. Not to mention the fact that it looks cool as well. Most tube preamps use cheap diodes for rectification and they may be followed by some SS voltage regulators. Some expensive tube preamps may use a vacuum tube rectifier, but most still use SS voltage regulators. The 5.1 uses all vacuum tubes for voltage regulation and rectification. The SA-2 also uses a vacuum tube rectifier and all tubes for voltage regulation. In fact, if you look at an SA-2 on the inside, it is very close to how the 5.1 looks. I had a CAT SL-1 Signature MKIII preamp and I sold it because I think the SA-5.1 is better sounding in a way that makes music seem live. That is the trait that Michael Elliot seems to have mastered with his better designs. They sound real. If the big knock against tube mc head amps is that they add some noise compared to SS or transformers, give me the noise any day because when the music starts you don't hear it and I would much rather have something that sounds like live music is being played in my room vice something that specs out great but dumbs down what is in the grooves and robs the life out of your system. Like I said before, had I not heard the 5.1/SA-2 combo work their magic and I didn't know what was really in my record grooves, maybe I could have lived with the transformers. Now I can't and won't.