I know you hate FR graphs that don't come from your friend, but I'm going to use them anyways since they are able to give a general signature. I need to get this out of the clear that many of the IEMs you mentioned are not flat.
The Westone ES5 is nowhere near flat.
The Westone 4R is actually a V with a midbass emphasis.
Same goes for the AKG K3003... It too is V-shaped
The Grado GR10 has a bit of a 3-4 dB dip to the midrange, and another 3-4 dB dip in the treble with a spike at the end that's nearly 10 dB in height. Special note on Grado, they don't actually fully have target curves to reach with their headphones. They listen to them. This is how they get that Grado signature (where they all sound extremely similar, but more refined). I doubt its linearity was a target, rather it did end up sounding the way Grado wanted it to, so he used it.
This one surprised me a lot. the Q-Jays as you say are flat have a deep V in them.
Phiaton's PS200 is definitely not flat as you'd think once again. It instead has a V-shape to it.
The companies flagship IEMs are not flat in most cases, they shape the music differently to create more properties out of it. These properties I speak about are the ones defined by the gloassary (detailing, clarity, sweetness, sparkle/splash, energy). None of these properties are defined as being flat, rather they are defined by having
shape. A suttle boost in the mid-bass causes punch and impact. A small spike in the 2-3k range causes a nice sweetness (I've noticed that spikes in the 1.5k can do the same, but have a slightly harsher effect). Timbre is the effect of having the main tone, as well as natural off-tones being in balance with each other on an instrument (this will create the most natural/truest timber as opposed to a more artificial one). I will state that none of these definitions state that a flatness in the midbass causes punch. A flatness in the 2-3k region causes sweetness. None of them, and they have reason to say this. A flat headphone will deliver all of these properties theoretically since they all are are small spike (but also a small recession at the same time; using some right and left limits and the idea that perfectly flat is unatainable). There is a way to get these properties, all of them with an uneven curve. I'll go further to say there are
infinite ways to do this. With every type of signature I've heard (bass heavy, V-shape, neutral, analytical, etc.), I've heard every single property be able to be reproduced, not in one headphone, but in multiple combined. This leads me to believe that these properties are not part of a certain signature, but are much more involved than that. Yes, certain signatures tend to have certain properties, but there is a list of properties that can be found in any pair of headphones iff it's tuned to produce it. This balance for the properties is so intricate, however, that a slight change to one will result in drastic change to another.
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