shifting soundstage>

pfrank, yes. Putting the speakers closer together will give you a slightly wider sweet spot as will moving the listening position farther away from the speakers. If a system does not have a sweet spot it is actually defective in some way. The more noticeable the sweet spot the better the system is. IMHE speakers that are more directional like horns and ESLs have a more distinct sweet spot. The problem then becomes that the high frequencies roll of dramatically as you move off center. Ideally the tonal balance should remain the same over the entire listening area. But, the image will only be perfect on the perpendicular line centered between the speakers. So if I want to check out the image and balance while someone is in my listening position I will stand just behind. 

Thanks to all for this very interesting and enlightening discussion which has offered much credible and helpful explanation.I now have additional information to conger while I am listening.

It can be changes in how the listener focuses while listening alone that results in shifts within the soundstage.