Again, very interesting John. Yes two speakers at the sides shouldn't provide as stable a center image for dialog, etc. as one speaker centered above(or below)the screen. What you have is actually a mini-phantom center formed by the two and the image should shift as the listener moves left or right, but not beyond the speaker on that side, regardless of how far off to the side the listener is. The same effect occurs with a regular two speaker stereo setup when it plays a mono source; as you move out of the center the image stays in front of you as far out as the speaker to that side. By the way, my preference remains with one vertical center.
