I'm beginning to suspect that my ears are significantly changing the sound signature of IEMs compared to people with "normal" ears because they sound completely different from what other people here are describing.
First, let me describe my ears:
I have very large ear canals and must use the L or XL tips that come with universals. The outside of my canals are a weird shape because I can only use very shallow tips; tips that are longer and more bullet-shaped (e.g. ADDIEM, Klipsche) are absolutely useless as they will NOT seal my ears under any circumstances. When getting impressions for my first customs, I also found out that the inside of my ear canals are much straighter than normal, with much less obvious bends.
Now on to what I'm hearing:
Most IEMs sound a LOT bassier to me than what other people are saying in their reviews. However, full-sized headphones sound LESS bassy to me than to others. For example, the bass on the Xears XE200Pro is generally described to be emphasized (but not ridiculously so) and doesn't bleed into the mids much, but to my ears the midbass was unbearably overwhelming and I could barely hear the mids at all. On the other hand, the over-ear M-Audio Q40 is considered a very bassy headphone, but to me they sound very balanced in comparison.
I'm starting to think that I hear full-sized headphones accurately because the sound reaches my ears in a relatively normal way, versus IEMs which essentially create sound chambers out of my ear canals, and because I have weird ears I hear IEMs weirdly.
Does this make any sense scientifically?