This speculative and off topic (Moderator please move).
IMO (yes, opinion) the value of harmonics is in smoother, more predictable, and less disruptive transitions, not audible distortion. It’s not about static states or continuous pure tones – modern digital video looks great with a nonmoving image but breaks into little fragments with rapid movement or a flock of birds. Old pre-digital TVs (CRT tubes and rooftop antennas) broke up in different ways, but never ever choppy fragments.
Systems with measurable audio harmonics sound to me as if they turn potential fragments into smoother audio blur. This means the tube amps that score at the bottom of ASR’s charts actually ‘fill in’ the gaps and often sound superior to ultra clean technical amps.
A fast, technical amp that measures well (e.g., THX family) can easily sound like sandpaper because time points 1 - 2 - 3 are fully split apart. Tube amps with excessive distortion sound awful/blown-out too, as 1 blurs with 2 blurs with 3. I posit that the low-level harmonics of tubes and the momentum/resonances of constantly moving dynamic drivers inherently smooth over the rough fragments.
I feel like deleting this message as I write it…ugg…