... As frequency goes down, ...
This has huge impact on the sound. Tonality ... timbre ... colored ... presence of rendered instruments ... physical presence ... clarity .... bass instruments now gets a rendering more similar to higher frequencies ... defined and precise.
I still do not see a connection to "
the score" which deliberately neglects the dispersion in this respect. It allows for wide dispersion as well as for narrow dispersion, dispersion may change gradually from one frequency range to the other. Only "the tilt" has to be maintained, namely a wider dispersion in bass to lower midrange, narrowing towards the treble, but not too much, and for sure not abruptly.
Your motivation to doubt "the score" is an anecdotal report on improvements, as is expected from every singled out experimenter in this field. Fair enough. But, not just for the sake of argument; I'm tempted to present a caveat from a scientific, truth-seeking point of view.
Let us assume the reference against which the improvements are experienced is a speaker which pretty much complies to Harman 's recommendations, a good score that is. A good score would ask for that tilt in the power spectrum.
The other, improved speaker has more narrow dispersion in the lower midrange, it doesn't get wider than the dispersion in the upper mids and treble. Hence it would radiate comparatively less 'energy' in that range, hence the power spectrum would be less tilted.
So the comparison is between two speakers of differing power spectrum, one being 'lighter', more focused on the mids, than the other.
"The score" says, such differences are a matter of taste, as people easily adapt to it.
You say nay, it is quality due to reflections and the like.
How do You differentiate an other, namely 'lighter' more 'energetic' tonal rendition against the influence of--let me summarise, 'reflections'?
Anecdotally I personally would say, that all Your descriptive terms I quoted above come to my mind, once I mute the bass / lower mids a bit, using a plain o'l equaliser, and listening just a bit louder then.
ps: in short, avoiding reflections in lower mid just changes the tonal balance, and it has nothing to do with phase/time/resonance and other obscure 'influences'
pps: even Your argument of a smoothed out frequency response cannot be trusted, experimental evidence is missing to begin with; think of the reasoning behind multiple subs: the more reflections, the better they tend to equalise out single, 'resonant' peaks and dips ...