Why are most High End Amps class A

Spatine, I share your respect for Nelson Pass and his products, even though they're not my cup of tea. But since his amps are generally single-ended designs with a high output impedance, I would say that the "class-A-ness" isn't necessarily the main source of their sonic characteristics, and the advantages/disadvantages of these aspects of his designs require separate consideration.

That's the main reason why I chose the Levinson ML-2/ML-3 for comparison . . . where the circuit designs are much more similar, whereby the Class A/Class AB distinction is THE big difference.

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.

I'll second the recommendation on Self's work . . . he also has a couple of excellent books on the subject. I don't necessarily agree with all of his approaches and conclusions, but everything he presents he does with wonderful concise logic, and thoroughly cites all his sources - which seems to be rare in the audio field these days. He's definately one of the modern masters of audio amplifier design, and a delight to read as well.

There's also Rod Elliot's explanation , if it hasn't been mentioned already.

http://sound.westhost.com/class-a.htm

Lot's of other articles/projects too.

I also have a Mark Levinson amp that has that AB class sound. To me the push-pull method has it's advantages such as clarity, efficiency, and low distortion. I've had this amp for ten years and I have enjoyed every minute of it... until now. Reacently I noticed that while the sound has the above advantages, it seems to lack depth and realism. A tuning fork has the same advantages as a class AB amplifier, but would you rather hear a musical piece played with multiple tuning forks, or musical instruments with their own distinct overtones. Class A amps have more distortion than class AB, but perhaps it's that distortion that gives class A amps their flavor, and realism. Anyway, that's my two cents worth.