I heard the Steinhem 3s briefly in 2019 at The Show in CA. But unfortunately, my aural memory of that experience memory is faded. I like the simplicity of the aesthetic design, and the build quality appears first-rate. As for the question as to differences with Magico, YG, Piega, etc., my sense is the sameness starts and stops with the chosen cabinet material, aluminum. Each uses very different drivers and crossover topology and components. Efficiency, as someone offered above, can be deceiving for various reasons.
I have found the ability of an amplifier to couple well to a particular speaker has a good deal to do with a speaker's impedance curve characteristics, as well as things like the transformer design, damping factor, and whether or not an amplifier is a transformer or DC coupled design, even an OTL, etc.
Regarding compatibility with low-powered tube amplifiers (especially SE designs), may be hard-pressed to find one capable of powering a modern multi-way loudspeaker to full effect. Contradicting myself, I happened to hear a colleague's Wilson XVS earlier today powered by Lamm ML3s (GM-70 tubes), and it was breathtaking on a wide variety of music. On the other hand, I understand the RB1 845-based David Berning monoblocks do pretty well with modern multi-ways, Einstein OTLs could be interesting, and I am also intrigued by the Alieno 300B amplifiers using a solid state power supply.
I do believe it's unfortunate that speakers and amplifiers get pigeonholed into pairings of use X amplifiers with Y speakers. Just look at the big brands, and I think you will agree. IMO what makes our pursuit so much fun is staying open-minded, sharing your experiences with others, and venturing out to listen until you find the combination that makes your heart sing.