Vertically oriented speakers are designed to have good of axis listening (horizontal dispersion), while horizontal speakers (most center channels) have good vertical dispersion and poor horizontal dispersion, which works well for mounting above or below sitting positions. I have done some non-scientific tests with my VP100/150 using only my SPL meter and my ears, the volume levels drop off by a db or 2 when you stand/sit outside of a narrow range outside of the horizontal center speaker. It doesn't sound odd or fuzzy just a lower volume.
Ideally your center would be the same as your mains, this is why Axiom now has the VP180 to better match the floorstanding speakers for those of us without the ability to use an M60/80 as a center.
