From Stereophile:
The Thiel CS3.7's voltage sensitivity is specified as 90dB/2.83V/m. My B-weighted estimate on its tweeter axis, assessed with DRA Labs' MLSSA system, was slightly above that figure, at 90.7dB(B)/2.83V/m. This may well have been affected by the Thiel's frequency response (see below). The sensitivity is usefully higher than average, which is a good thing considering that the CS3.7's impedance remains between 2 and 3 ohms over much of the audioband (fig.1), and that there is a demanding combination of 3.8 ohms and –40° capacitive phase angle at 60Hz. Thiel specifies the impedance being nominally 4 ohms, with a minimum of 2.8 ohms. I actually found the minimum impedance to be 2.4 ohms at 125Hz. The difference between 2.8 and 2.4 ohms is academic, either mandating use of an amplifier that has no problem delivering high currents.
Depending on which impedance setting you use on the M1.2 (I found the low impedance setting to sound the best) your maximum output current at 4 ohms will only be between about 7-10 amps.
Comparatively, my Aerial speakers have a similarly difficult impedance between 3-4 ohms (however, with a lower sensitivity of 86 dB at 2.83 Vrms and 1.0 meter on axis), and while the M1.2 reference amplifiers sounded good on those speakers, they ran out of steam too early. One thing I liked about the M1.2s is that I could essentially hear no strain right up to the point when they simply fell off the cliff and would not play louder, or more dynamic. I could have lived with them and been perfectly happy 90 percent of the time but instead I found that moving to a pair of very powerful (650wpc @ 8 ohms) monoblocks with bipolar output transistors provided much better control and overall sound with my speakers.
Regarding the sound, I am not convinced the hybrid design provides any more of a "tube-like sound" than would having a tubed preamp and SS power amp. I owned both the M1.2 Reference amps you are looking at and the older M1.1s and, while they displayed a nice organic, musical sound in my systems, there were other (solid-state) amplifiers I liked better. If you want a tubed sound, I suggest getting a tubed amplifier but you would need a pretty big one. Other opinions may vary.