I think that having music sound exciting and involving at modest volumes does involve overall system frequency response headroom as you describe.
however; it's more than just that. I think it starts with how responsive the speaker is to the particular amplifiers first watt......how much micro-dynamic energy get launched, and then how effective the mid bass and lower (but not deep) bass is. do you get the transparent texture and 'jump-livelyness' in the 50hz-150hz range at these lower modest levels? that's where the ease and vividness comes from....since it's the hard frequency range for any speaker and where most of the distortion is.
this is a Magico thread and I'm no expert on Magico. maybe they do this too. that said, on my amp/speakers I can watch the RMS watts both peak and continuous in a read out on the front of my amps. and even at fairly vigorous levels of volume typically it's '0' to 7-8 watts. at low levels it's at '0' to 2-3 watts. I think part of it is so much driver surface in my -4- 11" woofers per side (30hz-250hz) and 96db, 6 ohm load to the amp. I don't hear that same modest level 'magic' in low efficiency speakers with mega amps no matter the amplifier. something gets missed.
system frequency response headroom and modest volume level realism are related but not the same thing.....in my experience.
Mike
What your view meters are showing and what your amplifiers are delivering may be quite different .. As i have told you I am not a first watter. This said.. I am with you on the modest lSPL level magic with high sensitivity speakers. I had a thread on this on WBF too lazy to search and resurrect it.