Yes, well, I am a layman and a little bit of knowledge is a dangerous thing. So, my opinion might be entirely poorly educated speculation.
Not sure how EQ figures in directly, though it could alter frequency and timing/phase. Yes, the "mix" plays a role. But, if you start with a 2 mike stereo recording, what is there to mix? There it is just physical instrument placement, same as in mono depth. If you are mixing multitrack down to stereo, yes, relative front-rear volumes, and degrees of reverb/reflections generated by the instruments in the mix are the main cues as to depth. Mix in more reflections/reverb, and it sounds further away. But, I suspect this is all an oversimplification and there is still much more to it.
How do your ears tell the difference between the direct sound of the instruments and the slightly delayed reflections and reverb? Oversimplified, I think several things are important in that. Timing accuracy is crucial, because the reverb/echo is delayed and extended in time. There is a time and frequency spectrum of the reflected sound from any given instrument that is not the same as its direct sound, though the two are in some ways correlated for sounds from the same instrument. That is true in the venue or in the mix, as it is also in your room.
Directional cues are also important because the reflections and reverb are more diffuse than direct sound and they strike your ears from multiple different directions, even from all directions but unequally, in the hall, unlike direct sound from a single instrument. And, there are also perceived tonality differences caused by the interaction of direct with reflected sound. The same violin does not sound quite the same tonally when played in different venues or even in different seats in the same venue.
Again, it is complicated, especially when it seems so seemless, unconscious and natural when listening. I am unaware of any way to measure it, and its complexity would seem to defy objective measurement on a specific recording or in a specific playback system.