Reading again the OP, I have one doubt about Harman curves:
They reproduce the anechoic response of a flat speaker in a moderately reflective room, somehow an “average” room.
But why to not try to EQ to a totally flat response? If the ideal is an anechoic chamber (or am I wrong?), why don’t attempt to a flat curve in an apartment?
This is how the Harman curve is derived.
1. Start with a loudspeaker in an anechoic chamber. Tune it to measure flat at 1m.
2. Place the speaker in an ideal listening room. Measure the response from 2-3m away. You will notice a falling frequency response.
3. Swap out the omnidirectional microphone and place a listener dummy in its place. Now the frequency response will be modified by the HRTF. Comparison of HRTF with the Harman curve:
HRTF of a few individuals. You will notice a few things: there is a rising treble response above 1.5kHz, but how much it rises varies between individuals. This is because of differences in the shape and size of ears, head, neck, shoulders, etc.
Harman target for headphones (blue) and IEM's (red).
4. Tune your headphone to the HRTF target and submit it to a listening panel who can tilt the frequency balance to their preference.
The reality is a bit more complex than this. Pitfalls of the Harman target for headphones:
- as seen, above 1.5-2kHz the HRTF varies quite a bit between individuals. Also, the Harman curve is tuned to the B&K 5128 measurement dummy they are using.
- below 300Hz or so, the response can become variable depending on fit issues - e.g. adequacy of seal, clamping force, glasses, positional variation, etc.
Take home message: (1) When reading headphone reviews for compliance, don't read too much into it. Egregious departures from the curve (by more than a few dB) are unlikely to be preferred. (2) Compliance to the curve between 300Hz - 2kHz is the most important. (3) Always look at the distortion figures because it is more than likely you will have to tune the headphone to your own taste, preferably with your own in-ear microphones. I don't care how my headphones measure on a B&K dummy, I care what it measures like on my own head.