Why are most High End Amps class A

Class A amps run at full power when idling. So if you really want a dual use machine (home heater, and amp) then go for pure class A.
I have a small 50w 'class A' amp, and love it. I would never buy a big class A because of the heat, and strain on the electrical system.

Most Class A amps are higher priced, and not necessarily higher end. Due to the high heat output, most class A amp will need higher quality parts and much more expensive case work to dissipate the heat, thus resulting in a higher cost.

Class A amps will also waste a lot of power and run up your power bill, so be sure it's worth it.

Personally, when the time comes to look at a new amp, I will be looking towards Class D, which is the opposite end of the spectrum in terms of efficiency and value from class A.

Class A is the most linear mode of operation known. But- just like with anything high performance, is the exception rather than the rule, which is to say that AB amplifiers are far more common. On top of that, there can be so many compromises elsewhere in the circuit that they may well obviate the advantage of the class A operation.

Triodes are also the most linear form of amplification known, yet the same caveats apply.

The same goes for fully balanced differential operation, circuit minimalism, etc., etc. When you combine these elements, things tend to lean more in your favor, but execution is still an issue. Cost is a different matter- Class A amps do tend to be more expensive due to the larger power transformers required.

On top of that, equipment matching will still be an issue- see
http://www.atma-sphere.com/papers/paradigm_paper2.html
for more information.

Stanwal's advice is extremely good, and there are amps amps out there that will be a whole new world of resolution compared to the Mac, but not likely for the $$$...