+1 to mijostyn post ...
The third dimension is not instruments at different distances away from you, any system can do that and much of it is artificially created by the mixing engineer with echo. The 3rd dimension is the sense that and instrument or voice is a 3 dimensional object in space. If that space is full of reflections, echo and amplitude smear you will not be able to delineate the 3rd dimension at all. This is the state of most systems including some incredibly expensive ones.
I understand his point here but we must had the right balance between all surface (reflecting-diffusive and absorbing) then we can have too much room treatment if one of the three factor is unbalanced with the others for a specific room geometry size and content ..
A good live recording with a horn section is a great example. A top notch system will allow you to identify each instrument in space. Most systems show you the horn section, but you can not separate the instruments easily, the same applies to vocal sections. The spatial cues are usually there, but acoustic errors can easily overcome them because they are at a much lower volume. IMHO there is no such thing as too much acoustic treatment. An anechoic chamber is better than a poorly treated room.
I used my own mechanical equalizer with 100 distributed resonators located at specific places and this is more than passive acoustic treatment. We can create not only good imaging and holography ( the third dimension which is the sonic volume of each instrument ) but the end goal more than imaging done right is the listener envelopment (LV) Source width auditory (ASW) ratio.
But no, an anechoic room is a dead space , unnatural, it is better a bad room we will improve ... ☺😊