Excellence article. It shows that bad speakers don't get good over time, people just get used to bad sound. The small changes noted are far too small to be audible. The brain adapts to changes or imbalances that are input by the senses. I'm also an amateur astronomer and familiar with optics and their flaws. One example of how the brain adapts is the eye. The lens of the eye isn't achromatic and actually produces a significant violet halo around the edges of bright-colored objects. The brain, however, is able to adapt to this chromatic aberration and alter how things are perceived. Do you ever see well-defined blue halos around objects? Probably not unless you wear glasses which introduce their own errors. This is the same with sound. I guess you could say that the brain has tone controls and an EQ built into it.
As for soundstage and imaging that seems to have a lot to do with baffle design and especially room acoustics (as seen in the article) and are difficult if not impossible to quantify. If we could then one could easily predict or model how one speaker design would perform and how to optimize it. Then after a while nearly all speakers would be nearly identical if one particlular design or model worked best.
