Well the old Heaven series had similar distortion despite all the different housing materials that were used. Think it has to do more with the driver. Distortion is a big factor of what generates what we perceive as timbre, that comparison doesn't apply as IEMs are trying to generate a signal rather than creating a new one.Well doing the latter can be left more to subjective evaluation, 3rd order harmonics make it's sound sharper, seem almost sped up IME, Etymotics also have this to a lesser extent. Personally a distortion-less IEM like the RE400 has a much more natural timbre as IEMs with high distortion can make certain instruments sound kind of wrong, though sometimes that colored timbre makes certain other instruments seem more euphoric, but this makes things more specialized.
Really interesting. I'll be thinking about this when I hear the RE-400 next week.