Tbh Micro, genuinely I don’t see what the difficulty in understanding the distinction that an instrument sounds convincingly natural as opposed to not being natural sounding is... and also if what is discerned in listening (in what is after all a purely subjective experience) is invalid then what does any of this actually matter... and where is your treasured learning in this then if your experience validates nothing.
Denying what your essential experience tells you is going on (in a purely subjective experience) leaves you with nothing other than a handful of supporting specs and waterfall plots to validate what you are hearing and that there is a tired and insufficient old argument just waiting to happen (at best).
While I am unconvinced but realistically there isn’t really a problem that you might not get what the word natural is or that you don’t even seem to think that is an important distinction to make. Everyone gets out of this what they want. For me and (from what I read in forums and in reviews) many of us think is that instruments sounding convincingly as they should sound is a very basic, most essential and fairly fundamental capstone in the search for fidelity. It’s a test if the trick of music reproduction is working and using acoustic instruments as a benchmark in seeing if they then sound convincing is a fairly common and basic review step in how effectively a system is in portraying the experience of performance.
Final review in how effective a system sounds for me always lays with the listener and the listening and I believe many consider our experience of our systems and the music is in truth the very most and perhaps only important thing. So if you are genuinely unconcerned that when you play acoustic music that those instruments then don’t sound convincingly natural enough to sound essentially much as they are then listen to your potentially unnatural sounding system in calm and lovely bliss cause as everyone says there are just so many roads to both Nirvana and Rome. Tao out (in a most natural place and peace) in 2020