KuroKitsu
Headphoneus Supremus
Everyone perceives upper mids a bit differently because of their ear anatomy, both inner and outer, as well as ear/brain system. HRTF also comes into play. Versus speakers (what we'll call a natural base), IEMs are at a distinct disadvantage of sounding 'natural'. Sound coming out of speakers (or any real source) is altered by your body, face, outer ear, and ear canals before it reaches your drums. So, while you negate any 'room effects' below 500hz that speakers usually yield by using IEMs - and that sounds infinitely better BTW than speakers in a room - you now have to roll the dice on 800hz on upward because that IEM needs to match the very unique response you're ear drums are 'used to'.
That sounds very similar to what people describe as burn in, which I've always taken as getting used to a signature change rather than any physical change in the source change.
VERY expensive, you do take a bit of a loss everytime you plunge in to a TOTL and found something missing.Long way of saying.....you have to find the IEM that sounds most natural to you, or caters to your preferences while not sounding like garbage and retaining the technicalities we're all after. Unfortunately, it's trials and errors (expensive ones) - and the only final assessment that matters is how much you like them.