It doesn't appear there is a "standard" in audio for how to express soundstage quality.
I think this is the core problem... no standard way to express it, no standard way to measure it, and as a consequence seemingly not much in terms of ability to translate it back into specific attributes of a speaker system.
There is a school of thought that things like time and phase coherence in physical design and/or crossover design have a big impact, but that has not been my experience... if anything I find them to have more impact on the sound of acoustic instruments in ways that can not necessarily be explained by frequency response alone.
My impression from talking to various speaker designers is that things like diffraction effects off the edges of the speaker cabinet (and even off the edges of the drivers themselves) can have as much or more effect on imaging and soundstage... but that's getting well off the end of my knowledge.
