IIRC, @Mad_Economist suggested placing the microphone into an occluded ear canal, so I took the capsule from the Sound Professionals mic and placed it into a spare Symbio W medium tip that I had around. It’s not all the way into my ear canal, but pretty close.
Here’s how my curtain modded HD58X measures with that:
Intuitively this mostly makes sense, a very even response from 100 Hz to 10,000 Hz, with a bit of gain from the outer ear in the 1-5 KHz range, and pretty bad roll off at the extremes. The ear gain (even accounting for the lack of ear canal gain) does seem surprisingly low though.