My comparisons were done using the following components.
Klipsch Forte I, Denefrips Ares II, Erhard Aretha tube preamp, Leak CDT.
Room is about 12x25 with sound deadening due to furniture and normal house decor. If you clap your hand there isn't much if any echo. Music is mainly jazz. You can see a video demo of the SIT3 on my youtube channel under "drD E" or the link below. I will post more videos every several days. Obviously videos will never portray true sound but at least offer some sort of "relative" differences to provide clues. Use earbuds or headsets for best results. IMO, these amps are very nice but need to be used correctly with the efficient speakers so they're paired properly. Klipsch rates the Forte at 99db efficient so it's a great pairing. The Zen is getting a ton of traction due to recent reviews from popular YouTube channels. I think the wait time is well over a year. I've had this Zen for about 3 years and its the latest model. The SIT3 is rated for 18 watts at 8 ohms and the little Zen for 2.3 watts at 8 ohms.
My friend and I first started playing Cannonball Adderley, Pat Martino and several others. Jazz is a great genre of music for comparing amps because you often hear the sounds of individual instruments. Decware tries hard to emulate the sound of the Zen for all their amps. To fill bigger rooms and louder volumes, Decware resorts to push-pull designs but their main goal is to achieve the same sound as this little 2.3 watt single ended amp because it has a very unique sound signature that's very special when used right. The specs for First Watt are here for those interested. https://www.firstwatt.com/prod.html
The Zen is one of my favorite amps of all time for playing jazz and I've owned and compared many amps. This little gem has a very sweet, warm, airy and open sound signature. Whenever you hear flutes, guitar, piano and horn instruments, you get the sense of open space between each instrument. Even though instruments have amazing contrast with the Zen, it does have a very pleasant, laid back sound. First Watt amps are unique in providing a sense of tube sound, unlike most SS. The first time I heard the SIT3 with my system, I immediately noticed that it had a sort of thickness to the sound that was less delicate. It didn't quite have the finesse and sweetness that the little Zen had, but it had other things that I feel make it very special. The SIT3 added weight to the music and bass with speed that was able to keep up with the music when called upon.
Most of our listening levels were about 81db average which is a fairly pleasant listening for enjoyable jazz music late at night. Sometimes if pushed more, the Zen couldn't quite keep up with the volume of the SIT3 but as long as we played at moderate listening levels, the little Zen was extremely pleasant to listen to. Most solid state amps I've heard struggle to provide an open, airy sound but First Watt amps are really quite remarkable because they really have a tube like sound, but with more darkness. Tubes can sometimes have a sort of grainy sound, particularly newer models like the ones flowing in from China.
The trend with these imported amps is often a bright, detailed sound signature that is far from the way a Zen sounds. The Zen is not grainy at all. It's very smooth and very sweet sounding and if you are into smooth jazz, it's an incredible performer and one of the best amps in the world for that IMO. Both the SIT3 and Zen sound perfectly balanced and equal in that regard. The soundstage of the SIT3 and Zen seemed similar but the SIT3 can really take the volume to higher levels cleanly without distortion that can really open up and give a bigger sense of sound scale for bigger rooms.
IMO, the SIT3 is a much better amp to couple with open baffle speakers than the Zen because it really has a sense of weight or heft to it. Open baffle speakers often tend to have less meat on the bone and the SIT3 and First Watt amps in general provide additional meat. The Zen doesn't quite have the full heft of the SIT3. It sounds like a smaller amp and it is. I've also listened to the First Watt F3 in a different room with other speakers and although I have not spent time with it using the Forte I's yet, my prediction is that it will be an even better match than the SIT3 for the Forte. To my ears, when listening to the F3, it sounds more tube like IMO than the SIT3 and it's not quite as dark sounding. I think the way components are matched will play an important role as well as the genre and volume levels intended, so I don't think either amp is necessarily better than the other.
The Zen did provide more finesse and space around individual instruments that was so beautiful to hear and had a prettier sound, but the SIT3 was also very nice sounding and each amp had something the other didn't. This is why I think it's better to collect a few amps so you can use them according to your listening mood. Hope this helps others. Both of these are world class performers IMO.