Yes, I have had my Musician III back from Spectron now for over a week, maybe going on two. I went from Musician III to M III SE Mk2.
Differences are:
Bass is more controlled than before. I know this doesn't seem possible but as the amp is burning in I am finding the bass to be more extended, and articulate.
Speaking of articulate, voices, speaking and singing, are more clear by a magnitude. Easier to understand, even background singers and massed choral singers are not just understandable but clearly so.
I use a horn based speaker system to cover the mids to low treble. I never really gave the horns much thought. They were always clean and present, but now they are, like the bass, more controlled and faster sounding. Cymbals are shimmering in decay in a better way. Pianos are reverberating in a better, more focused way. Also the integration between my horns and ribbon super tweeter is better integrated. It is as if the horns have speed up to near the same speed as the ribbons. It is a pleasure.
Also, there are some physical differences. The two huge capacitors are replaced by banks of smaller, quicker ones. The output wiring is JPS Aluminata, and the binding posts are Cardas Patented ones. Now I use a bare wire connection and still these binding posts are better--better grip and easier to use. I also think they may sound better, but there is no way of distinguishing this with all the interior changes to the output section. Also, the fuses are different. Using upscale ceramic fuses have always made a change for the better, and I am sure in this case, too.
The SE Mk2 is in every way a better amp than the stock Musician III. I am damn happy I spent the money.