Well I tried passing on this timbre discussion, but I guess I just couldn't resist
The reason why there is a such a different opinion between whether a 'neutral', as in lack of warmth, or a warmer tone is more accurate when it comes to timbre, lies in what is perceived as more
realistic. Adding warmth to a signature results in a smearing of transients. When a note starts to sound veiled, there is a loss of sense of purity which is associated with coloration. Especially when it comes to string instruments, increasing the note articulation (lower treble around 5-7 KHz), the attack of a note, improves the sense of how 'pure' it is perceived. Further adding a sense of clarity to that note by boosting upper treble (around 12 KHz) increases the sense of how clear it shines through. So, there are two ways to improve this sense of purity of a note, which automatically gives it a feeling of realism or trueness; making its attack harder or its tone brighter. In that regard warmth does the opposite, which is decreasing that sense of purity. Brighter iems (Galaxy, RE800, Dream etc.) therefore always tend to sound good for string instruments.
Even so, when you hear a real life string instrument, there is no sense of brightness. It is a lightly warm, 'natural' sound; even an instrument like a violin or a piano, which tends to sound better in iems when it is reproduced brighter. But by definition, an actual instrument of course also sounds very
pure, since it is right there in front of you. The practical problem for any audio system is that these two are very hard to combine. I've had a similar discussion with Rick once when a musician came over and played a $20K acoustic guitar for us. Even when we are listening to one of his $50K 2 channel systems, the ones that can most accurately convey that sense of realism, all the micro-dynamics of a chord being struck, still fail to reproduce the tonal beauty of the actual guitar. Or perhaps,
especially the ones that are geared towards that sense of realism. In the end there is always just an innate conflict between purity, and the warmth required for an accurate timbre, when it comes to realistic reproduction of audio.
However, I would argue that when it comes to the construct of
timbre, not realism, a slightly warm tone is required to sound accurate. There is no reason why a signature that lacks warmth should be any kind of objective standard; the only benchmark is comparing to real life instruments. So one might make a case that a warmer signature sounds less true, but that should be seen in the context of realism, not tonal accuracy. Fact of the matter is, iems as the 5-Way, Phantom, and Layla will sound 'dull' for many people, which is fine of course. Everybody is entitled to their own opinion, and more importantly preference. But that doesn't mean they don't sound natural. In fact, sometimes us people that prefer warmer signatures are also aware that it sounds dull, and wouldn't mind some more sparkle or clarity. We simply endure this dullness because we would rather have a more natural sound, even if it is less exciting.