First...THANKS Eric....for so darn many thought provoking threads that you have started!
I agree with the hemisphere/half bubble description. No doubt that often the slightest change in speaker positioning will affect the overall sound...sometimes significantly.
And, the straight line, arc, hemisphere soundstage is maybe most significantly influenced by what the recording engineer did with ambience, echo, reverb and delay....and then how your speakers and room interpret/project what he intended.
For a long time, I was obsessed with the soundstage...but realizing that perfection is probably an illusion in this area, once I had a good stage, I turned my attention to tone...especially piano, drums, horns and voices.