Harman's 2018 Over-Ear Headphone target versus Oratory's 2021 Diffuse Field compensation curve, in case anyone's interested....
To make the difference a little bit easier to see and contemplate, I've traced over the two curves. The Harman curve is in black, and the DF curve is in green.
Imo, Oratory's DF compensation curve is probably a bit too depressed in the low treble, and maybe also a tad too bright in the 2 to 3 kHz range to accurately reflect the DF response of
his specific gear, pinnae, and measurements.
The DF curve above is probably not completely accurate for the rig that Harman used as well. (To the best of my knowledge, they never published any DF measurements for their specific rig and pinnae.) The upshot is that you have to take some of the differences in the levels between the two curves, esp. in the upper mids and low treble, with a grain of salt.
The Harman curve (in black) is also generally too rolled off in the high treble to be considered neutral in that range. That is why a few of the headphones on the above plot are excursing above the Harman target in the high treble. That is normal imo for a neutral headphone.
The dip at around 9-10 kHz in the treble is also a normal feature for a neutral headphone on Ora's plots imo. And I believe a neutral headphone should also have a bit more of a dip in the upper mids than the Harman target, in approximately the 1.5 to 2 kHz range. That is why most of the headphones on the above graph are sagging down a bit in that area.
I chose the 5 headphones above specifically for their general similarity in shape to the Harman curve btw. That does not necessarily mean that I believe they have the most neutral responses though. Because I don't think the Harman target is completely neutral. (And I'm not referring specifically to the bass.)
Imo, the headphone on the above graph that is probably closest to a neutral response is the Senn HE-1, which is a very expensive e-stat. And the next closest might be the Onkyo A800, which is a dynamic driver headphone that is no longer in production (and has an unusually wide headband). The Onkyo is an open headphone though, so it rolls off significantly in the bass below 50 Hz. And the HE-1 probably drops down a bit more that necessary in the sub-bass as well. It is also an open headphone btw.
The dips or notches in the low treble at around 4k on the Beyer Custom One and AKG K371 are
not normal features of a neutral response imo, btw. The notch on the Beyer is so narrow though, that I might just leave it as is. And not bother trying to correct it. And the two bumps in the bass on the Beyer and PSB are also not neutral.
Ora's graphing tool used for the above plots:
https://headphonedatabase.com/oratory/headphones