At this stage you would use the program Sinegen ( http://sinegen.en.lo4d.com/ ) to adjust the high frequency response, since your post-EQ results will almost certainly not match what your ears actually hear. To do this, use an image of whatever target curve you wish to use for reference. Let's say hypothetically we're EQing to diffuse field curve:
After loading your EQ settings in EAPO, sweep through 1000 - 20,000 Hz with Sinegen while listening, listening for obvious peaks and valleys and keeping in mind your reference curve. So, if you're using diffuse field curve, it should steadily increase in volume up to ~3 kHz then steadily decrease in volume after that.
It seems to me the graph ^ that we are using as a reference for the frequency sweep, is just the diffuse field target curve, rather than a perceived loudness curve (for a headphone calibrated to the diffuse field target).
If I'm right, then what is the use of 'keep in mind the reference curve' when doing a loudness sweep in sinegen, if the reference isn't a perceived loudness curve?
You said that 3kHz is meant to be the loudest frequency. Are you using something else entirely to determine that, like an equal loudness contour? Is that what I should be using as a reference instead?
doing that with an equal loudness contour in mind for a gross idea of how we're just less sensitive in the upper and lower range, that can work well indeed after getting used to doing it. but just trying to even things out so the all sweep feels flat, that's not neutral at all.
If you do need to use an equal loudness contour for the frequency sweep, then for example, it would show that 1kHz and 2kHz should be the same perceived loudness, and also there should be a small dip at 1.5kHz. Should I equalise so that there is no dip at 1.5kHz, or should you expect to hear this slight dip on a neutral headphone?
In summary, I'm finding the sinegen frequency sweep very difficult, since I have little idea, or any reference, of how either a harman or diffuse field target (or any kind of neutral target) should sound perceptibly in terms of loudness of pure tones.