OB Design for use with small SETs

[Max N]

I thought I would split this off from the Quasar thread and the 'Economical OB thread'.
My personal interest in building a pair of OBs stems from hearing what James' Quasars can do when driven by SET amps, including Jonathan Noble's little silver wonders as well as NickG's 211 uber-amps.
Just to be clear, one of my motivations for considering something similar, rather than re-building my IPLs, is so I can play with small SET amps.
The other threads contain some interesting stuff about OBs, but the Orion's are active and the DarkStar project seems to major on active crossovers and SS amps. Both are valid approaches and may well be excellent. I am not trying to be prescriptive about what people post, but I do think it will be easier for other people in the future to pick through the various threads and find stuff that is relevent to their plans if we divide the threads.

So this thread is intended for discussion of high(er) efficiency OB speakers with benign impedance curves suitable for use with smallish SE amps.

James, as this topic evolves, I am starting to get the impression that a wide-bandwidth OB with high efficiency is not an easy thing to achieve......
I am starting to appreciate that your Quasars are perhaps quite unusual in what they can do?
I have no experience with OBs or full-range drivers. It would be great for me (and others I hope) if you could pen a few words about how you arrived at the design of your Quasars.
Or, to come at it from another angle, what are the design criteria/compromises/difficulties when trying to design a wide-bandwidth high-efficiency OB?

Cheers

Max