I had a S5.5, purchased from Mike at Audio Archon. Great guy to deal with. Had the newest and improved transistors according to Doug Dale. While their website and documentation leaves much to be desired, Doug answered all of my questions and concerns in a timely and respectful manner. It sounded very articulate, clean and detailed and easily drove my Selah Audio Ceramica speakers (6 nominal 4 minimum, 88 dB), however, musically it just never got my toes tapping. Beautiful amp, great build quality, never got more than a little warm, great confidence in its reliability and potential longevity.
But I believe it would have sounded much better with a solid state preamp instead of my BAT VK-51SE tube preamp. Its XLR input impedance is only 10kOhms, so most likely too low and not a good match for an XLR tube preamp. Just wanted to point this out to anyone potentially considering use of this amp with a tube preamp. It sounded better with my Lyngdorf MP-40 as the preamp, but not even close to the overall fidelity level of the BAT tube preamp with the 2 amps I tried with over 100 kOhm input impedances. So I sold it to TMR and it sold within a couple of weeks. Of course, your results may vary with your tube preamp, but I can say, unfortunately, that I found it to be a mediocre match with the BAT tube preamp.