I understand the way of thinking.
except.... diffuse field is hardly the same as a speaker in a room.
In DF the calibration is for sounds coming from all directions in equal loudness (in an anechoic room) entering a specific HATS with a specific pinna, ear canal + coupler.
Speakers in a room have sound only coming from the front (under a small angle) and reflections in 'a room' (which most likely is not every one else's room).
Those reflections are arriving at a later moment and the brain can tell. The microphone can not.
This thus is not a proper representation of what is actually heard but mimics the sound power picked up by the microphone so for measurements this is correct.
Headphones do not use the room nor does the sound come from all directions. Pinna activation is very different and sound is coming from the sides only in a planar way.
Also tactile feel is not taken into account and the rise of lowest frequencies in your average room in the DF compensation.
So all in all the DF compensation for speakers (measuring sound from all around that is equally loud with a specific frequency dependent directional and stereo phonic direction is hardly a correct target for headphones.
If it were Harman curve would have been very close to DF which it isn't.
Newer HATS with newer standards are a bit more accurate but whether we like it or not are still not accurate to human hearing but are, kind of 'averaged' and adhere to (newer) standards which is handy as they (and other HATS) can be calibrated (compensated) so measurements on HATS-A and HATS-B using the their specific calibration to a specific standard are comparable.
The newer ones are closer to 'reality' than older ones with ancient couplers and tubes as ear canal.
Nope... DF for a HATS is not suited as a target for headphones but understand your line of thinking.
I appreciate you taking the time to write such a detailed reply on the above, solderdude. Imo, you give some of the best input on this forum on a consistent basis. I don't think you really are grasping my thinking on this though, based on your comments above.
The arguments you're making are along the same lines as those made by Harman, when they decided early on in their research that diffuse field curves would not be a good target for their headphones. Based on your comments above, maybe it would surprise you to hear that I actually agree with both you
and Harman that the DF curve of a measurement rig does not make a particularly good target for a headphone's raw in-ear response.
I've actually tried equalizing the raw in-ear frequency responses of some of my headphones to match the diffuse field response of the rigs that were used to measure them. And they sound too bright. If you do this, what you'll probably end up with is something that might be closer to the sound of a loudspeaker in an anechoic chamber, with all of the reflections and room gain in the bass
removed.
This is not what I'm suggesting though in any of my posts or links above.