I too find this region to be sibilant. I have created custom eq settings that I'm happy to share. However, these settings are what work for my particular pair of he400i and ears. Remember everyone has different size pinnae and ear canal depths, so slightly different frequencies will resonate with each user. In My case 8.3k actually needed to be boosted very sharp to hear it and surrounding frequencies (avoiding a large grainy gap). However, here are my settings FWIW. Hours of sine sweeps went into this, and it's still not perfect, but so much improved over stock. I'll also snap a screen shot so you can check the relative curve to your eq plugin. These settings require a relaitvely high Q value, so not all eq software will be able to replicate this. Melda Production Mequalizer is free and handles these sharp Qs very well.
EQ settings:
https://docs.google.com/spreadsheets/d/11fj14v1I5KA1Vuqgv7mlez4HXZMYrHOvlYxqaRFhaN4/edit?usp=sharing
I find the balanced gain setting to be the most neutral to my ears - I deactivate the 3K filter to get vocals and guitars to be a little more present.
Pic Mequalizer:
attached to post - if you can't view it (for some reason I can't) I've also attached it in the spreadsheet link above
let me know what you all think.