I own a pair of Sonus Faber Auditor M's and I use them in my secondary system. Sources are a Basis Signature 2500/ Dynavector IV/ Lyra Titan 1, Mac server and Wavelength Cosecant Version 1 USB DAC, & Jolida JD-100A. Accuphase P-3000 amp and Shindo Auriege pre-amp. Auditorium 23 interconnects.
Like you, I found the bass response to be far better than I expected from the specs. This was true for the auditioning and for using them in my primary and secondary systems. And this involved/involves rooms with highly variable dimensions and numerous other quality sources and amp/pre-amp brands and types (tube, SS, hybrid, even integrated).
I love their sound, which I perceive to be somewhat on the warm side of neutral, but for me, still very musical.
One thing I noticed during the multiple auditions and subsequently in my two systems is the fact that these speakers really start to shine when they are provided with considerably more power than the minimum power suggested by the specs.
Out of curiosity, what other speakers are you considering against the Auditor M's? I chose them over Vandersteen Wood Quattros and DeVore Silverbacks, but it was not an easy choice. I found the Silverbacks to be a little too polite. The Quattros image better than any speaker I have heard, but I found the Auditor M's to be more musical. Of the three, the Auditor M's were the ones that I found to be the most musically involving/satisfying. Their far better than expected bass response was the other factor that allowed me to choose them over the Silverbacks and the Quattros.