OK, thanks for the explanation. I'll take your word for it (I don't use MDR-7506, little motivation for me to spend time testing what you are saying), that listening to MDR-7506 gives a narrower experience.Basically these headphones have a hyped center image which strongly masks the sides for whatever reason. The best way to explore this in comparison to other headphones would be through a VST plugin by RJStudios called True Mid/Side which allows to separate or alter the volume of the "phantom center" image independently from the sides while leaving the sides where they are in the image (rather than what a MID/Side encoder would do by squashing the sides together into the same channel). So what I mean is that the phantom center of these headphones to mean is extremely dominant and causes the stereo width to be more narrow than my other headphones. The HD650 and Slanotes Zero I have present a much wider stereo image in comparison.
I also don't think this was his motivation, but a side effect of the width of their stereo image which benefited his mixing style.
It's also fun to try a cross-feed emulator with them, as you were suggesting with how speakers operate, because the result is not a massive difference in comparison to the results with such a plugin for other headphones.
That said, I'm not sure that jibes with your complaints about mixing with them. (Again, I'm not advocating mixing with them.) Are you concerned that your mixes will be too wide, because you'd be compensating for the perceived narrow width? If so, OK, that would be you doing you. As for as Scheps, mixing on speakers, which has been the gold standard forever, is a narrower experience than any headphones, so I'm not sure a narrower image is a negative for Scheps. It might be closer to mixing on speakers, for him?
(Note that I've never advocated MDR-7506 for general home listening pleasure, only that there are reasons they are popular for recording. Mainly for monitoring while tracking, but as long as you have them and are used to them, then it's whatever else you want to do.)