Regardless of the similarities and driver configurations in center channel speakers compared to the mains, what you are describing in terms of what you are hearing is the inherent limitations of horizontal center channels. Adjusting your ear level is essentially the same as re-positioning the speaker and center channels have always been more sensitive to this issue.

Back some some time ago Alan Lofft wrote an interesting article about this when comparing the VP150 and 100 and how the sound changed just by a slight move in location.

It seems nothing will ever beat the original THX concept of identical speakers in the front three channels, however, when you have big "floor-standers" in your L/R configuration, for many it is not possible.