Frogman, thank you kindly for the implicit compliment.
I essentially agree with all of the foregoing comments, including yours. As Phaelon and Robsker have indicated, generalizations about design approaches will inevitably be wrong in many cases. But to the extent that there may be a TENDENCY for separates to produce bigger soundstages than integrateds, I would expect that in a lot of cases a major factor is that the more circuitry is put on a single chassis, the more opportunity there is for unwanted interaction between various parts of that circuitry.
In principle it would seem that that tendency could be overcome with sufficient care in the design process. And I don't doubt that in many cases it is overcome. But any design involves so many competing variables, tradeoffs, and compromises that it wouldn't be surprising if the inherent and potential advantages and disadvantages of a particular design approach tend to come to the fore more often than not.