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.

Shadorne - careful topology is always important but I think that as long as transconductance in biased area is non linear extending this area doesn't alleviate the problem but makes it worse (extends nonlinear area).

This problem is different from crossover distortions or switching distortions. Of course huge amounts of negative feedback will make everything nice and linear but for the price (TIM) and even then still not as good as class A. The only strength of class AB is that it costs less and therefore better quality amp (power, parts etc) can be made.

Class A amps have finesse, ambience, subtlety, blossom and decay of sound that gives it a more realistic quality. However, they are the most expensive amps per watt, the hottest running, uses the most electricity per watt output, most tempermental, requires the most attention and upkeep of all the amp types. I say that, because most pure class A amps use tubes...until recently. Check out this month's issue of Stereophile Magazine and you will see a review on one of the most recent amp models from Pass Labs. It's a pure class A SOLID STATE amp which got rave reviews. I am referring to the XA- .5 models. It is very hard to have a high powered pure class A amp which is why the high powered ones cost many tens of thousands of dollars. However, the 30 watt stereo amp, and the 60 watt mono amp are each only five thousand dollars. This is very reasonable for the quality of these amps.

The wine analogy that Kirkus used might be the way to go here, but with wine the you can try many types easily and see what you like and have a collection of Cabernet, Pinot, Chardonnay, etc.

But with Hi-Fi, you can't collect to the same degree, and you cannot sample to the same degree. So I think for many people we rely on the audio equipment equivalent of the master sommelier, like reviews from Stereophile as stated above or sound engineers with a lot of experience and people who have been into it for a long time. The expense of audio, and the rate of audio collection, generate a reliance on expert advice in this kind of situation.

So what is the advice for the guy who started this thread, he likes his McIntosh gear. What would be an interesting flavor change, would a Plinius SA100 mkIII be an interesting taste choice? Better is pretty subjective, but how would a class A amp with his McIntosh pre be different?

My days of sampling different amplifier classes are over. I don't want to knock anybody thus won't mention various equipment brands. Yet I can tell you that through the years I listen to equipment from a lot of major brands. None of them remotely come close to what the real Class A amplifier can do. As to those who have other priorities to spend money at this moment, it does not cost anything to just go listen to these equipment. When you see that much a difference, perspective and priority can develop different meanings. Because of the type of discussions and reports people have about the XA.5 series and that I own and know how Pass Lab XA60.5 sounds, it's obvious as to my recommendation. I am just too in awe of the difference to sit still on this.

Now from a different angle, the more expensive a purchase, the longer it better stays with me. Even $100 is a good amount of money, needless to say $1000 or $2000 that Gp_phan is thinking about. I am not about to spend $1000 when realizing that a year from now I would loose so much just because I really want something else instead. But that's just me.