I have those. I found them to be a tinny mess -- kind of the opposite of a solution to OP's overwhelming sibilant problem. They can be EQ'd to sound impressive (and indeed I do crave listening to them from time to time), but they suffer with profound grit when playing heavy polyphony and complex voicing. I wouldn't recommend them for mixing, even with EQ.
@Supercurio I don't think you're imagining the sibilant peak. See the measurements at
https://www.innerfidelity.com/images/JVCHAFXT90.pdf -- but definitely take them with a grain of salt. Measuring ear phones isn't an exact science, as ear canal shape and length varies from person to person. These variations can have significant effects on response at the ear drum. Also, Tyll Hertsens (the InnerFidelity measurement guru) is still refining his head-related transfer function compensation curve, so the peak you hear could be more or less pronounced than what you see graphed. Be that as it may, the native uncompensated response should be sloping down at 5 kHz, not up.
You could try putting a snip of a paper towel over the driver hole and securing it in place with the silicone ear tip. That ought to attenuate some of the sibilance, and would probably make them sound quite nice.