According to UE's marketing material, the UE18+ has an entirely reworked crossover network, and the implementation of the TrueTone drivers and acoustic chamber that were introduced with the UERR. I can't be remember how different they sound, as I haven't heard the original in years, but those are the definite technical changes.
Dude, I don't know why it looks like we're disagreeing here; I absolutely agree with your central argument. The only thing I'm adding/arguing to your initial point is that it
is two sides of the same coin because it is two different ways of interpreting the oscillation; objectively through measurements and graphs, and subjectively through the human ear. The information received by the IEM remains the same and the information output by the IEM remains the same, the only difference is who's on the other end.
I also don't get your reasoning as to why they're not two sides of the same coin, because what you're implying seems to be an IEM that's been measured to perform a certain way will be heard differently by different people due to how the brain interprets the information it receives (which is true to an extent). But, my argument is: How is that different from inconsistencies between measuring rigs? As proven by the initial argument that started all this, different rigs (people) with different coupler shapes and quality (ear canal shapes and critical listening ability), insertion depths (again, ear canal shapes), microphones (eardrums), software and hardware behaviour (brain-and-ear relationships), etc., exhibit different readings that have to be normalised and run multiple times in order to actually gain legitimacy. I can even flip the argument and say that if two people have the exact same opinion of an IEM (to the point where if they each wrote a review of this IEM, they'd write them verbatim; word for word), and it turns out the IEM measures differently on different measuring rigs, are the two people then objective and is the measurement now subjective? The only reason why I consider human impressions subjective and measurements objective, is the terminology used; audiophile speak in the former (which is inherently arbitrary but can be normalised to a degree (as shown in Nic's shoot-out)), and numbers in the latter. But, despite this, they each have their own caveats when it comes to reading and understanding them, evaluating their legitimacy, etc., such that they both have to be taken with the same grain of salt, which renders neither of them absolute and unambiguous... to me, at least.
I think this is all I'll say on the matter, I've dragged on this tangential discussion for a wee bit too long
I hope you can see where I'm coming from. I'm not necessarily disagreeing with you; I'm just, in my opinion, supplementing your points with my own point of view. Cheers, man!
P.S. Based on my experience with mono recordings that have absolutely no soundstage apart from probably depth cues due to their one-dimensionality, they will definitely have no soundstage regardless of the IEM you play it through. Of course, differences in frequency response, among other things, will cause the recording to sound different through different IEMs, but will there be a sudden artificial soundstage? No, absolutely not.
