That's a diffuse field measurement, and dBr is a relative measurement. Relative to what is anybody's guess, my guess would be unequalized response of the headphones. More likely however it doesn't mean anything at all in this case. There is a reason why the E2Cs have this frequency response, it's no engineering mistake. You can try cancelling it out but you probably won't be amazed with the results. Diffuse field can harm as much as it can help, it varies greatly based on ear structure. I'm aware these are canal phones, but your perception of sound is based on ear structure regardless. It wouldn't make much sense trying to EQ it unless you know exactly what you're doing, a job like that will require crafty parametric tweaking.
The best thing you can do is to use the Steinberg externalizer.dll VST. As I said, this is a VST, so you will need to accompany VST support in whichever player you are using. If it's a portable player you're probably out of luck, portable players are unfortunately ******. Otherwise it could greatly enhance your listening experience, in fact I'm convinced it will. The plug-in implements the necessary crossfeed all headphone plug-ins must have and it does a particularly good job at it. From what I remember of my analysis of it's spectral filter, it also implements what is known as a 'pinna notch' which is a simplified version of the diffuse field in your headphones. Simplified is good in this case, but things might get a little too 'notchy' in your situation. See if raising an 8kHz band some 10dB helps, and I mean really helps, not make believe helps. And by raise I ofcourse mean relative raise, as in don't actually increase 8kHz gain, but decrease everything else, and by this I don't mean drag all EQ sliders down, I mean reduce the preamp gain on your EQ to compensate so you don't clip. If you're confused, it's really not worth thinking about it too much, just implement the VST and leave it at that, the results will be satisfactory in themselves. As to how to implement it, that depends, look into VST support for your player.
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