I can post a FR of the K872,
from the issue 2017-04 of Stereoplay, and I want to say a few good things as I allow myself to post their work: Stereoplay is the most reliable hifi magazine I know; I understand their FR measurement curves of headphones are corrected such that the FR would be like a speaker FR (i.e the ideal FR is a horizontal line, not the Harman-Kardon line) , making it much easier to interpret.
The top curve is the FR, the bottom curve the distortion :
Notes:
- the date indicated is from November 2016, so I assume the version tested is a production model with the foam insert
- strong bass, corroborating that it would be a model with foam
- big and steep hole in the FR at 3 kHz with a huge distortion, indicating one of these strong resonance effects on which EQ does not work [ in my experience, EQ works when the recess has a rather weak, slow inclination, not a steep fall ( this slope being relative to the fq position in the FR as its scales are not linear but logarithmic ) ]; I personally hear issue with voices which may correspond to this hole at 3 kHz in the FR, but don't mind as I am not mainly using my K872 for vocal recordings.