LOL. Just stop. Mudding the waters. The treble on sennheiser headphones specifically the 600 series have major treble deficiency and the high mids/low treble is shaped wrong. Get a proper way of understanding measurements and stop the bullshit.
For example, in the microphone world there are similar people who dont understand audio gear. On the left we have a at4040 microphone with a 6khz peak and 8khz cut. The treble is all screwed up. On the right we have a TLM 103 with a smooth clean treble. it is exactly the same with headphones.
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Here we have the dt990 burn in measurements. All the peaks in the high frequencies legitimately smooth out over time. Wide soundstage. Very smooth treble. Bass emphasis. Dynamics are good. Then we have HD600 series completely screwed up in the highs especially.
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Sennheisers hd 600
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You are using Diffuse Field still? Pure diffuse field has some issues to start with:
1. You are using a fully reverberant assumption of how transducer and room interactions combine, which is not even true in the speaker world. Both free and diffuse field have issues with their underlying assumptions of how real listening and professional rooms are set up, in other words, they do not account for the existence of both reverberant and anechoic aspects that rooms have. Trying to translate this into a headphone target where even more constraints exist is worthless since the basis of your target is deeply flawed in the first place.
2. Both curves heavily rely in measurement equipment that has not been standardized. No measurement fixture and microphone have an exact frequency response match with even an average human response. Measuring these targets twice will yield different values with different measurement equipments, and how do you know which one is correct?
3. It does not account for the perceptual and psychoacoustical characteristics of humans.
4. Which version of DF are you even using? What fixture was used to measure and what were the room dimensions and acoustical treatment? How many samples were taken? Many other questions just specific to your compensation curve may be asked just to make your case of the validity of your curve, let alone how the headphone was measured and the characteristics of the measuring equipment and methods.
In a personal note, I have a pure DF EQ for my headphones, and subjectively I can tell you that it does not sound even close to speakers in a room. Free field is even worse.