The very notion that a designer starting with state of the art drivers can achieve perfection has an implicit "begging the question" problem--it assumes that there is such a thing as a state of the art driver and that driver has the potential for delivering perfect sound. Even taking "perfection" out as a goal, there is no such thing as drivers that represent state of the art in all aspects of performance; there are only drivers that are good at certain aspects of performance and remain workable in those aspects that they are not that good. Sometimes, the very thing that makes a driver a contender for state of the art in one aspect of performance (such as the dynamics as transient response of the Manger bending wave drivers) makes them particularly difficult to integrate with other drivers in a coherent fashion. The craft of designing and building speakers is not simply putting together the "best" this or the best that, it is almost the opposite--it is working around and ameliorating the problem area of each component. Even when that is done well, a particular speaker will have inherent limitations--it will sound its best only in a particular room location, it will sound good only to someone who shares the designer's taste and priorities, etc.
There is no such thing as a consensus choice of a "best" crossover design--the best design is a matter of taste and priority, the characteristics of the driver, the characteristics of the speaker cabinet and a whole host of other issues. Also, many crossovers are not merely dividing the frequency spectrum between drivers, they are correcting for frequency response issues, driver phasing, "baffle-step" issues, floor bounce issues, etc.
The best cabinet design is, again, the best for the particular drivers, the best for what the designer is trying to achieve sonically, and that does not necessarily mean something braced so well that it is sonically dead. If that were the case, designs like those of Harbeth and Audio Note would never sell.
What Nigel needs to learn is that design in not simply a matter of taking the best from column A, B, and C to slap together a speaker. He must go the much harder route of determining what sound he hopes to achieve (with the understanding that MANY compromises and trade-offs will have to be made); experimenting with the almost infinite combinations of drivers, cabinet designs, and other design factors, and finding a way to manufacture the final design achieved after all that hard work.