SET amp comparable to First Watt SIT 1?

Yup, true. I've used OTL and SET on a very wide variety of speakers under optimal conditions for both amps. I'll put my statement cited in further perspective:

40, 30, 20 years ago, I would have (and did) recommend and own OTL tube amps for maximum transparency, speed and musicality, used with appropriate speakers, including over all the SET amps I'd gotten my hands on up to 1999. That's when I first happened upon an Audion SET 300B amp. And then further put original KR Enterprises 300Bs and well-chosen NOS input and driver tubes in it. With the emergence of objective high efficiency speakers (ala Zu) in the 2000s, the higher power of OTL became less necessary.

The Atmasphere amps are certainly transparent and fast, and I don't dispute the rise time measurements you site for them. But Audion SET amps are uniquely fast and transparent examples of that topology and I hear them as a notch above OTL.

Phil

A friend of mine is a happy owner of the Atmasphere MA-1 amplifier and MP-1 preamplifier. He has efficient and easily driven speakers ( so amplifier power wasn't a determining factor). We compared his amp with my 300b SET and no other system component was changed. Both of these amplifiers were "very" transparent, open, fast and clear. Both had exceptional staging, imaging focus and music flow/pace. The SET playing the same recordings was fuller in tone and body with an increased sense of presence. It presented harmonic information that was more vibrant and a bit richer.The Atmasphere was relatively more towards a leaner sound (but by no means pale,sterile or flat). Different people will prefer one or the other. I happen to be drawn more to the fuller instrument sound and nuance of the SET.  Both amplifiers are very fine but speak with their own sonic character.
Charles

tmare - fwiw, I used the SIT-2 and SIT-1 in a large space like yourself and like to even crank electronica at times. I would recommend more wattage like 213Cobra suggests.

I think if you are considering SS, you might look at Pass XA-30.8 which you can audition from Reno Hifi. There also is a pair of Sophia Electric 845 amps on this site which can get you into good SET relatively cheaply.

@charles1dad  I'm good with that- that's how I would describe it too. You expect an SET to sound fuller as it has more 2nd harmonic distortion where the MA-1 really does not have much at all. I think that if you had a sound pressure level meter in the room, you would have found that although you were careful to match the volume of both amps by ear, that in reality the MA-1 was actually playing at a higher level.

The leaner sound our amps often have is simply that they make less distortion (most of it being the 3rd harmonic, at about or slightly less than an SET will make at the same percentage of power level with respect to full output).

When designing any kind of amp its all about the distortion it makes as to how it will be regarded. For example solid state amps have a lot less distortion but what they do have tends to be higher ordered harmonics which the ear translates as 'bright' and 'hard'; for a solid state designer its the 7th harmonic that is the one to be avoided if possible. I figure those are the drop dead harmonics to avoid if you plan to play the system for hours and hours on end. That way you won't get fatigued.

Atmasphere,
Yes , I've heard the distortion argument rendered many times and that may in fact explain to some degree what I am hearing.What is interesting is the fact that the SET consistently sounds more real and closer to what I experience at live unamplified venues. That's why I just follow my ears and choose what sounds right and more life like. Again we all will listen and make our personal choices whatever direction that may be.
Charles