Why are most High End Amps class A

tests show that underbias and overbias causes increase in distortion

Agreed but I don't understand the point? Of course you have to match the bias precisely between both sides as otherwise you'll get loads of higher harmonic distortion. However you can design circuits that help achieve this and help maintain this under various conditions - it all boils down to careful topology.

the ML-2s were like a high-maintainance chick that looked gorgeous on your arm, but didn't know how to have fun when you got home and turned the lights out.

Kirkus,

How about a design that runs Class A to two thirds power? Is that like a gorgeous chick that also knows how to have fun when the lights are out?

And you're correct in the assertion that increasing bias doesn't improve the problem, it just increases the signal level at which it occurs.

But it does reduce the audibility of the problem. You now have distortion only when the output signal is very high and you have no crossover distortion (GM doubling or whatever you like to call it - lets say transition distortion) when output signal is very low and it runs in Class A (both sides conducting).

A small absolute amount of distortion on a large signal is better than the same absolute distortion on a small signal.

In one case the listener may notice the transition distortion (large part of the overall signal) while in the other case it will be much less audible due to it being a smaller proportion of a much larger signal.

If anyone is interested there is a master class lecture by Douglas Self on audio amplifier design. His talk is interesting for its lengthy discussion of distortion mechanisms and the new Cambridge 840 XD design which he mentions briefly (a kind of offset bias to bring the crossover region to where it is less obtrusive).

He did not mention active speaker designs with limited bandwidth as a solution to the many necessary compromises required to achieve wide bandwidth in conventional approach- but he talks about nearly everything else.