It's really more about the authentic timbre of (acoustic) instruments. A tenor sax and a flute can both play A=440Hz. They have different overtone structures, and specific instruments will have slightly different sounds, when played by different musicians. Overtone structures involve multiple frequency ranges, so in a multi-driver system, more than one driveer is contributing to the "sound" of that instrument, playing that note. This is where all those little differences add up to a big deal. Our ear-brain systems are pretty sensitive, and can detect these differences, i.e. when the speaker, headphone, or iem is changing that delicate overtone structure. This is what people refer to when they use the term 'coherence'. (Or, at least, iin my opinion, what they should mean/be referring to).
Single-driver B.A.s, or most headphones, have an advantage in this coherence race, so long as their drivers are not sufficiently non-linear to mess it up. Lots of so-called full-range drivers either sacrifice bass or treble extension, in order to maintain that univocal coherence across a broad mid range of frequencies- typically these are the ones with the most natural and organic timbral performance - the instruments really sound like themselves.... (This is my personal Holy Grail, as if that isn;t obvious by now). What is also interesting is that, pyschoacoustically, if the phase response is good - i.e., the overtones line up properly, the brain will fill in the missing fundamental, or upper harmonics... In other words, it's almost as if you can hear bass that's not there, and it makes the 'speaker' sound fuller.
I have also noticed that musicians often "listen" to gear differently than audiophiles. Musicians listen for technique and hear through the filter of knoowing the mechanics of playing (that) instrument, what it takes to make those sounds. Authenticity of tone is less important, since the sound is just a jumping-off point to hearing technique/chops. (This is neither bad nor good-it just is.) At the end of the day, the gear is there to create pleasurable cognitive states in the listener, and whatever brings you bliss is wonderful.
Right. The characteristics of each frequency is what makes everything so different. Kind of like how a voice or instrument could be the same frequency but sound very different from another. Harmonics and resonances within both the instrument and ear canal can change so many aspects. There's no doubt that multi driver systems the reason why people desire for multi-BA systems. The idea as you mentioned is one of the advantages for going with said solution. By splitting up the work between multiple drivers you could effectively produce more vivid and realistic tones while producing more clean output. However, as you also mentioned, crossover systems are quite difficult to make perfect and I am willing to bet tolerances for such are quite strict.
However, single driver systems automatically claim the outright advantage because of the reason you mention. What is interesting is you see the same with audio as with visual. We don't view everything perfectly but its our brains that predict and fill in missing information assuming there is enough to go about it.
I guess if you wanted to, in a perfect world scenario, the multi-driver system will win. But the problem is that achieving that is much harder said than done. I have no clue about how to build crossovers so I can't say much about that, though I do enjoy learning about this stuff.
I'm a musician/audiophile hybrid so I guess its why I am so conflicted. Nah, but I'm a musician as I have been playing instruments since I was very small. I presume, its why my hearing tastes and experience varies so much from other audiophiles. Regardless of which is better, both are relatively excellent systems when implemented well. In this case, which is these high-end IEMs, you really can't go wrong with either systems or the hybrid. For me, I feel DD is better than BA as for whatever reason I "feel" the music. It sounds different than any of the BA drivers I have owned and I find it more my tastes. Good exchange of info though!