Oh really? Well, this will raise a few heckles!
This post talks about humbler pieces of equipment than those mentioned above but I think what I'm hearing is still valid.
I have a Marantz mcr603 which has class D amplification and which has performed brilliantly for the 5 years that I've had it. The sound with my Yamaha wxc-50 and Elac debut b5 speakers is unfatiguing and engaging, not the last word in detail, but amazingly good considering the price. At the Bristol show I heard the Audiolab 6000A and got bitten by the upgrade bug and I recently picked a newish one up for much less than rrp.
As I totalled my computer last week (an errant cup of tea!) I've been restricted to listening via aptx bluetooth using Amazon music HD on my phone. Before people say that listening like that is not using the amp to its full potential, well yes obviously, but I reckon the quality is equal to if not better than 320kbps from my computer over wifi - bluetooth has come a long way. Anyway, what am I finding? Well the Audiolab is digging out more detail and the sound is bigger but the higher frequencies on the Marantz/ Yamaha combo just sound sweeter and more natural. There is a strained quality to the upper mids on the Audiolab, most noticeable with female vocals and brass instruments. This surprises me given that the Audiolab has more power.
A word about imaging and sound stage, this is where my set up really might be compromising the Audiolab. My room is such that the speakers have to be placed quite far apart, about 3.5 metres, and quite close to a wall (about 10 inches) so getting a good soundstage that the 6000A is famous for is not really possible. The other acoustics of the room are pretty good - quite high ceilings, not too many reflective or hard surfaces, wooden floor with soft furnishings etc.
Anyway, what's my take away from this so far? Well, I think class D amplification is far from being the "devil of audiophilia" and matched well with other components can sound pretty good. Obviously, the real test will come when I get my computer back and can stream over wifi. It could be that the Audiolab is exposing the shortcomings of my Elac speakers in the higher frequencies, they are renowned for being a bit lacking in treble, but I have always liked their sound signature.