Since the Mirage OMD's have been mentioned...Andrew Welker who now works with Ian at Axiom was the designer behind those beautiful Mirages. There is likely patents that Mirage has on that particular omni-directional design, so, even though Andrew was instrumental in it's concept...
Back when these "Omnidirectional" speakers were first introduced I was quite intrigued and ended up purchasing the center channel speaker that was part of the model group that went with the floor standers shown in the ad. It had two 5 and a half inch woofers with the same tweeter positioned over the small mid-range and even though it was limited by its relatively small size thus producing the usual "boxy" mid-range sound, it had, by far, the most amazing off-axis response I have ever heard in a center channel.
You could literally stand 90% off-axis, facing the side of the speaker, and the response was almost the same as sitting directly in front of it in the normal sweet spot!
Regardless of size, off-axis response is always variable to some degree in any "front-firing" center channel so I would be interested to get a response from Andrew on this and could a similar design be configured in the Axiom models that could produce a similar result as the OMD design?