From Peter W. Mitchell and published in Stereophile Magazine.
Just as I said in previous post, the major advantage of wider separation between speakers is in improved ability to resolve instruments close to each other. Well, that's sort of like what I said, and I was certainly thinking along those lines. And he says that adjustments of this angle are among the most important thing that can be done for playback realism.
Reveals that the technically correct angle for recordings made with a coincident microphone is 90 degrees. However, that does not work well with recordings made with spaced omnidirectional microphones; a "hole in the middle" develops. For those recordings, an angle 60 degrees might work better. And notes that many audiophiles and even dealers in USA use angle lower than 60 degrees in the range 45-60 degrees. 60 degrees is a commonly accepted quasi-standard "compromise" listening angle recommended by many books and articles. That is simply when the speakers are as far from the listener as they are from each other.
So concludes that moving listening chair forwards and back depending on recording is not a bad idea. Peter himself simply leans forward and back in his recliner, covering a distance of 18 inches, depending on recording.