My theory is the the way various amps make distortion has a lot to do with it. I used to think it was how well the amplifier was able to get phase relationships right and I still think there’s something to that, but by no means is it the only thing! One thing that seems to have a bit of correlation is the decay in harmonics as the order of the harmonic is increased. It needs to be on an exponential curve. Another thing that I feel needs more research is the role that the 2nd and 3rd harmonics play in allowing the ear to make out the detail required to reproduce the soundstage correctly. This relates to my first sentence in this paragraph. If the amplifier isn’t devoid of distortion but the 2nd or 3rd is too low, apparently that can prevent the amp sounding 3D.
IME the class of operation has little to do with how well the amp is able to winnow out soundstage depth and detail.
thanks ralph for your thoughts, i tend to agree it is about higher order harmonics in the distortion profile and how these affect the auditory sensation of the decay of notes