Were any Tannoy-type dual-concentric designs examined? The amplitude charts supplied with my 8in DC2000s are smooth and within a few dB across the range, apart from a designed-in 'fashion' bump around 50Hz. These drivers must surely have been improved upon by now?
I would be interested to know your directionality test method, to check that aspect; they have always been spectacularly sharp in their imagery, but I have never been able to decide if their sensitivity to positioning is due to their ability to accurately 'probe' the environment or if the units had some funny characteristics of their own. I think they have about +/-45deg dispersion with essentially no phase or frequency problems, deteriorating beyond that.