I also own the First Watt M2, which I bought several years ago at a very steep discount as a backup amp. I would plug it in once in a while, thought that it had good things about the sound but also a bit wonky and gamey for some reason.
M2 is basically a voltage gain auto former with a single pair of MOSFET per channel and no feedback anywhere.
I almost sold it but decided to give it a full court audition instead of the occasional 'toss it in and listen to it cold' routine. I left it on for a week, and boy, it really woke up. I don’t know why maybe it is the open loop design, but it might just need a lot of break in and galvanic and temperature stability to open up, but open up it did. I’m glad I kept it and now use it for the time being as a STAX headphone amp in Santa Cruz, where over many months it continues to improve with use.
M2 is a hard amp to describe. It seems to be one of the favorites of the DIY crowd amongst the First Watt designs released to them, with some of the SET/Horn crowd adopting it in lieu of their toobs.
Where the M2 and the VFET part company, they are already at a very high standard of detail, sound stage and imaging. VFET veers off towards absolute separation and black spaces with deepest tone retrieval. M2 goes toward a kind of whole-ism and organic connection to the sound tapestry, with great PRAT (if you find any meaning in that acronym). M2 does a strange thing on speakers. When sound, especially grouped sound, gathers up and moves louder, the whole sound stage seems to light up to give a vivid image of the performance and venue that kind of flashes off into space. Upper mids have lovely gloss and sheen. M2 has perhaps the best rendition of live strings and massed strings I have heard in my system, not at all solid state and very convincing.
It seems the new SIT3 is an effort by Pass to harness a bit of each of the SIT/VFET trait and the M2 trait, because it also uses an auto former voltage gain stage with a very unusual push pull output consisting of a SIT and MOSFET in tandem, very carefully chosen to match characteristics.
If the SIT3 can succeed at merging the qualities of SIT and M2, it must sound like quite something. That’s why I am curious to hear SIT3.