Why are most High End Amps class A

Most of you are thinking of 1 type of class-A power amp - the type that Krell, Plinius, Threshold, Forte, Sugden have made/are making. This type of class-A power amp does dissipate a lot of heat as it is biased to run full power bias current all the time. The heatsinks on these amps are large (they have to be!) & the current draw is high thereby running up your electric bill.
There is another category of class-A amps that use sliding bias. I believe that Symphonic Line & Karan amps (there might be others but I do not know) fall into this category. These amps run cool when there is no signal or when the signal is low (from my experience I would say when the SPL level at the listener's chair ~9-10' away is in the 80dB range). As you crank up the signal input, the dissipation increases markedly. These amps do not run up your electric bill.
Just FYI.

As others have pointed out - a well executed class-AB amp can be very satisfying sonically. I can say this from my own personal experience.
However, [b]THERE IS[/b] a certain sweetness to the sound from a class-A amp. I realize that more when I take out the class-A amp & put back the class-AB amp. It's much easier for me to discern what I lost (when I put the class-AB in) than what I gained (when I put the class-A in). In my particular case I say "lost" because I personally like the sweetness of the class-A sonics. YMMV.

but it's Achilles heel is not that it's Class B . . . but it has a quasi-complementary (all-NPN) output stage. The reason for this is that when these amplifiers were designed (in the late-1960s), complementary NPN/PNP power transistor pairs simply didn't exist. What this means is that in your amplifier, the basic linearity of the output stage around the crossover area is quite poor.

Kirkus, I'm having a very hard understanding why an all-NPN output stage is very non-linear at the x-over point. Please explain. Thanks.

In addition, the driver stage is really primitive, with simple resistor networks setting the current through the input differential-amp.

There doesn't seem to be anything primitive about this input stage except that it is (very) low gain since resistors are used as the load to input differential pair.

I think a good upgrade for you might be to get a later McIntosh amp, from at least the MC2255-era (early-1980s) or later. By this time, they were using fully-complementary output stages, input stages with current-sources and current-mirrors, and voltage amplifiers with an active load -- meaning they're an order of magnitude more linear.

Kirkus, what do you mean by "linear"? And, how do "input stages with current-sources and current-mirrors, and voltage amplifiers with an active load" make the power amp more "linear"?
Thanks in advance for the insights!

Thanx for the explanations, Kirkus.