I think there are 2 major factors that separate the best speakers from the good speakers, and 1 major factor that separates the best speakers from the great speakers.
I think all good speakers can image the sound very well. The good speakers will lack bass.
Some great speakers may have the great bass, but they lack that last drop of prestine resolution and detail, and some of the instruments sound like they are barely in the background, being overwhelmed by the drums and other bigger louder instruments. They will image a great 3D soundstage.
The best speakers will have great bass and the best resolution/detail - meaning you can hear every instrument being played. They will image a great 3D soundstage.
But is that worth the $20K difference? It seems like such a minor detail for such a major price difference.
I disagree with the great Audio Critic guru Peter Aczel on one thing. He thinks that only dipole speakers can image the best 3D soundstage - that all monopole "box" speakers are monkey coffins.
I think monopole and other non-dipole speakers can image just as well as the dipole speakers.