Why is there this "push" to Class-D in the first place? What do you gain? In what way would they ever be sonically better than a good Class A or AB amp?
@moonwatcher Class D offers something that is very hard to achieve with A and AB amplifiers: a very high value of Gain Bandwidth Product. Most solid state amps use feedback, but as you might know, feedback has gotten a bad reputation in high end audio, not because it doesn’t work, but because it usually gets poorly applied- and so causes distortion of its own, adding many higher ordered harmonics (to which the ear is keenly sensitive and interprets as harshness and brightness).
So the answer is pretty technical. One reason feedback has this bad rap is because when the GBP limit is reached, feedback decreases on a 6dB slope and perhaps faster with succeeding octaves. This can and does happen as low as 1KHz, so distortion will begin to rise- putting higher ordered harmonics in the most sensitive region of human hearing. What I’m talking about here is easily measured: distortion vs frequency.
When you have enough GBP, distortion vs frequency can be a ruler flat line across the audio band. This allows the amplifier to be smoother in its presentation, with greater detail at higher frequencies (since distortion obscures detail) without harshness. Smoother and more detailed at the same time is a good thing IME.
Class D makes an enormous value of GBP available to the designer. So you can run very high amounts of feedback without getting into trouble; we’re running 10x more than conventional A or AB amplifiers. This makes possible an amp that is smoother and more detailed than conventional A or AB designs (even in tube embodiments), eliminating the benefit that class A used to offer.
The reduced heat, reduced size, weight and cost are all nice side benefits.