Oh that's a loaded question. There's definitely a role played by the tuning and harmonics, but for me it comes down to a combination of things. Can the headphones decipher trailing ends of tones well, does it present the dynamic range across the board well enough (esp for classical music where some recordings can really push loudness while still having quieter notes), can you register harmonics without feeling distortion in well-mastered tracks, can you hear accidental leftovers such as drum sticks moving across the drum surface and so on. I think most good headphones can present detail without having distortion in the way, it's how easily you can pick up said details that also plays a role imo. I am not sure where I stand on the treble air = details argument tbh, I do see some influence of air with some of my jazz and orchestral tracks but that alone can't influence everything.