Why are most High End Amps class A

Bombaywalla - not only bias but also gain (before feedback)is different between class A and AB. Class AB requires much higher gain (usually about 10x) to linearize output transistors with deeper negative feedback. Large gain causes amplifier to overshoot (and even choke) on fast changing input (pulse) since it is unable to feed signal back in the same phase (delays the signal). This is called Transient Intermodulation Distortion and was dicovered in 1970. Before that first SS amplifiers had excellent THD and IMD but unpleasant sound (overshooting introduces odd harmonics).

Using different classes of amps for woofer and tweeter is fine if we can guarantee that phase shift is the same.

Most amps are NOT class A, but AB

Kijanki, could you please explain about the phase shift in a bi-amp setup? Are you saying that different amp has different timing as to when the music is projected out?

Spatine - I've never had bi-amping setup but noticed that people, most of the time, use identical amps for that. Absolute phase should be the same unless amplifier inverts the phase 180 deg - easy to fix by reversing speaker wires. Problem starts when you get amplifier from different family. My amplifier, for instance, is a class D creature based on Icepower 200ASC module from B&O. Data sheet for this module shows +40deg phase shift for all frequencies (0-20kHz). I'm not sure what to think about it. Rule of operation is different and this amp shouldn't perhaps be bi-amped with an amp from another family.

It would be cool though, to use class D with its excellent power, dynamics and bass control with small class A amp for the highs. I'm afraid that harmonics would add improperly and sound would be strange.

Standard (class A, AB, tube) amps might have slightly different absolute phase shift (and different interaction with the speaker) and I wonder about using different amps (or different family of amps) for bi-amping.
Had anybody done that?

GP_phan, whether or not an amp is ostensibly "class A" is probably the last thing you should be worrying about. True Class A operation is extremely inefficient, and while it works well -- it's expensive because of its inefficiency. Which means that the term "class A" becomes a very abused one in audio nomenclature -- kinda like "digital". That is, it seems like everybody wants to say that their amp is CLASS A in big type, and then put in little type how it's not really, actually a true class A amp.

But specifically for your situation. The MC2105 is a very finely built amplifier and will last many years, 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. In addition, the driver stage is really primitive, with simple resistor networks setting the current through the input differential-amp. I'm not faulting McIntosh for these design choices -- they were simply doing the best they could with the materials and experience they had.

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. And if you like your 2105, you'll LOVE a 2255.

The C28 is a mixed bag -- its circuitry is very much like a tube preamp, but with transistors . . . so it's fully Class A, single-ended operation. Its weaknesses are that the high impedances used (like a tube preamp) make it a bit noisy, and it's pretty complicated and failure-prone in interconnection and grounding. Specific common failures are dried-out main filter caps (causing oscillation), and that four-section volume control gets noisy and has channel-balance problems when it gets older. You might get it serviced, but if yours seems to be working well, then I'd probably upgrade your amp first, and keep the C28 for a little while - and eventually upgrade the C28 with a late-model unit like a C36 or C38.