I'll chime in, too, as I've owned Sound Labs for several years and also am a Sound Lab dealer. The Thors may likely be a good match with Sound Labs, depending upon how they can handle the complex load. Other than that, I can report that a substantial 100 W/ch tube amp had no trouble driving my U-1s which employ the current technology. I've heard of 150 watt tube monoblocks driving MBLs (efficiency in the low 80s at best) extremely well, while some other 750 watt monsters, also tube, fell apart trying to drive the same speakers. Quite a different speaker load, but maybe you see my point that you can't rely on wattage ratings.
As already mentioned, Sound Labs remain a tough load, but the higher efficiency technology used beginning this year allows a wider range of suitable candidates for amplification.
The improvement in efficiency on my Sound Labs was noticeable although I didn't measure it, but the increased detail and responsiveness are at least as noticeable. I suggest these latter improvements are at least as much a result of lower distortion in the backplate components as they are of lower amplifier distortion.
I've owned several other ESLs through the years. Very few could match the Sound Labs in some areas, while none of them even came close to Sound Labs overall for for a completely satisfying music listening experience. I purchased the Sound Labs as a consumer and ultimately decided to become a dealer for them.