Back in the days when I was listening critically (the 1980s and early ’90’s), I used a pair of 15 ohm Rogers LS3/5As. I had them sited upon a pair of bar stools and canted inward so that their axis was just in front of my listening area. Back in their heyday these speakers could image better than any conventional enclosure speaker I had ever heard. IMO, only the Quad ESL could image better.
Happy Accident
I've always read that the distance between the speakers should (more or less) equal the distance from each speaker to the listening chair.
Our system is in the living room and due to furniture location, I'm guessing the former distance has always been about 10% less than the latter (math is not my strong point). Experimenting with toe-in has resulted in either a strong center image with a narrower sound-stage or vice versa. I've assumed the only way to improve matters would be to move my listening chair forward or the speakers further apart, neither of which is possible.
But, as my wife will tell you, audio brings out my OCD tendencies. A couple days ago, I was once again messing around and tried toeing-in the speakers so their focus converged about 18 inches behind my head. This meant that the distance between speakers was, as always, 9 feet and the new distance between drivers and their point of convergence was 12 feet.
As a result, the sound-stage now extends significantly beyond the speakers on each side without the center image collapsing. Actually, it's harder to detect a "center image", as the "image" spreads right across the sound-stage. The sound-stage is also deeper and resolution and bass actually seem to have improved. The point of convergence is only inches in front of a large, Craftsman style book-case with the typical "lattice" design of glass and oak. The speakers flank a slate hearth.
Lacking the sort of experience/knowledge that might explain why "breaking the rules" has paid off to such an extent, this scenario falls into the category of "happy accident". I can only guess that it must have something to do with the room.
Perhaps wiser individuals could offer a more scientific explanation...