Just a few comments because I was not there.
This is very unlikely to be an adequate target because DF is an unnatural and largely impossible acoustic condition. Olive made roughly the same comments in his papers about why the Harman headphone target became necessary.
Gunther Thiele would say that headphones are unnatural and a largely impossible acoustic condition to begin with
- and argued in favour of using DF HRTF.
Just to be clear we're now talking here about using DF HRTF as a baseline, not as a target. On top of that DF HRTF baseline are added preference adjustments.
But conveniently, whether DF HRTF or another reference sound field such as Harman's "in room flat" is most appropriate is, in the case of the 5128, a bit of a moot point apparently, and generally speaking the targets that use the 5128 DF HRTF as a baseline + preference adjustments similar to the ones Harman added to their "in room flat" curve tend to produce, with the sort of coupling insensitive headphones Harman tested the original target with, up to a few kHz, quite similar error curves anyway.
Somewhat more probable, but in the end the targets are not dissimmilar from each other and there's not much that can be expected from headphones that have not been individually EQed using in-ear measurements. We know LF region will differ in tilt, curve and boost by a few dB, and on a person susceptible to leakage effects and preference. MF will be flattish on a fixture and on a person. HF will be elevated due to ear resonance but totally dependent on the dimensions of the ear and the final tuning based again on preference within a few dB.
Harman already did the baseline research about subjective preference and basic requirements (such as preventing high-Q resonances), and produced a defensible target. Beyond introducing a requirement for individual EQ and changes to source material standards for music presented through headphones, I don't know what other progress we can expect. Changing the shape of the curve by a few dB is not important. What would be much more valuable is if Harman produced recommendations about new headphone designs that show better FR stability across many people.
The test concerned IEMs. Indeed the most preferred curves are quite close to each others, possibly closer than previous studies Harman conducted, which could have contributed to a noisier result.
Harman IE 2019 is quite unsatisfying to me from a theoretical standpoint : it requires an explanation as to why individuals would want, on average, their bass
and mids (and particularly the latter) to measure differently at their eardrum between in-ears and over-ears, and in a rather
specific pattern. Some of the other targets discussed in that talk suffer less from that problem if not at all - but so much for the theory as Harman IE 2019 was at least as well liked as the alternatives proposed here.
I'm also increasingly skeptical that we can envision the idea of a target for passive IEMs (and most active ones as well) quite as we do for coupling insensitive over-ears (ie the ones with low inter-individual variation, good leakage resistance, and low positional variation), for reasons related to fundamental in situ response vs eardrum HRTF mismatch IEMs suffer from anyway (I'm thinking for a start about two variables at least here, ear canal length and eardrum / middle ear impedance variations), but that might be another discussion.
