I will try and find the schematic. Its a lot simpler then you might imagine.
Yuichi also has a 1.5 to 2 inch adapter for using 1.5 inch drivers (people with Tad 4003s are not complaining..except for the price)
What you have measured about these alternative Be diapgragms is consistant with what I have been advise by company that uses Truextent diaphragms in field coil compression drivers with a Tractrix horn, hence they use a Be super horn tweeter above 16 khertz. The tractrix horns are apparently subjectively better then the Tad 4001 horn but you need a super tweeter.
So don't throw out your 2405s yet.
I am inclined to think the 0476 with the bi radial is still the winner.
What this means is as usual there is no free lunch but you can have fun making it work.
The notion of a flat response to 20 khertz is more important on paper to the Japanese than it is subjectively anyways provided the diaphragm is not in breakup mode too early.
A gentle tappering off above 10 khertz can be a good thing in some situations
I think Greg has mentioned this before.
By the way I got some complaints from the local natives attempting to run a series external ground plane measurements with LMS. 2.83 volts at 110db sensitivity above 10 khertz is freakin loud if you are a dog I guess.