I am convinced some stereo can be perceived way outside of the triangle you mentioned. It is probably no "effective" stereo (whatever that is exactly), but a residual perception of the spacial stereo dimension in a sound.
The link below is a recording of a wedding I had in 2012. My recorder was placed on the organ gallery at the opposite side of the church to where the singer and I were placed, at a distance of at least 130-150 feet. My speakers were placed on one keyboard stand, only 3 feet from each other. But nevertheless, I can still detect a slight stereo in my piano sound at this distance if I listen to the recording with headphones. It would sound different if my system had been mono. (BTW, back then I used the BK-7m).
https://soundcloud.com/rorosetree/hochzeitsmusik-somewhere