Actually, Gavin, I think that honor belongs to Diffuse Field or OW tuned IEMs. The 1plus2, as great as it is, doesn't match either of those.
The DF and OW curves were derived from plotting the perception of a flat loudspeaker, and iems like the UERM, ER4S, F111, etc attempt to match that in the mids and highs. This generally involves lifting between 1 and 4K, areas where the 1plus2 slightly shies away from to create that sense of space. I'm sure Inks will be here shortly to expand on the matter. After all,
I think the difference here is, you are looking at it from a technical stance, charts and graphs.............For me it is just about the music and how close a system gets to reproducing that performance. I listen to a lot of live un-amped events and my goal in this hobby is to try with the components that I have to get as close as possible to the feel of that event, be it from IEMs, headphones or speakers.
Plus no speakers are flat once they have to interact with surrounding...........it is all a fallacy!
I really have no interest in technicalities, although I have been looking at charts in my professional career for 15 years and the only ones that worked for me is the point and figure chart..........not because the chart is good, but many of my peers who moves the markets with sheer volume, were using the same charts. And it is basically just reflects price action........