Heya Bob O! Let’s talk some about headroom in respect to distinguishing “categories” of loudspeakers. I personally believe you can get away with loudspeakers with less than superior headroom for mixing. If I recall, for a time, Eric S. was using Genelec 8040A fairly close and not too loudly for mixing. So he did not exercise the known headroom limitations of that small active loudspeaker. Of course, Eric could get a great mix with two tin cans and a piece of string 🙂
But my point is that I personally agree that GOOD headroom is important for mixing speakers. But I also say that SUPERIOR, the best headroom is critical for mastering speakers. In my opinion, mastering engineers should seek to use loudspeaker/amplifiers that do not self-compress at performance monitor levels. Short term Peak SPL capability of, say, over 110 dB at less than 1% THD from 100 Hz up, and less than, say 5% from 50 Hz down. Which is very rare in the mixing world, but I think should be de rigeur in the mastering world. I think for many genres, short term peak SPL of, say, 100-105 dB at less than 1% THD from 100 Hz on up is acceptable headroom for a mixing speaker. Less than 5% from 100 Hz on down in that case. Anything better is desirable but not essential for most mixing genres, in my opinion.