Based on what I’m hearing from experts I’d certainly consider Class D on my next purchase. I think most layman, like myself, pick one up and it just feels flimsy, so how could it be as solid as 90lb beast of an amp from the past? I think I’ve been conditioned to think heavy=better build quality and parts. That said, I would challenge higher end manufacturers to have the beautiful case work and aesthetics that many lack if they choose Class D designs.
Weight and size is largely a function of power supply and heat capacity.
Power Supply
Your old amp recharged the power supply capacitors 50/60 times a second (Hz). Those capacitors store charge as voltage peaks at that rate, then provide the energy to produce power (watts, sound) between charging cycles. The transformer needs to be big to supply enough power to keep the power flowing at 50/60 Hz, and the capacitors need to be large (in value and thus physically) to supply enough power to the amp between recharging peaks.
A modern class D amplifier's power supply operates at several hundred thousand times a second, say 200~500 kHz, so recharges the power supply capacitors about 5,000 to 10,000 faster (more often) than older "linear" power supplies. That means the capacitors can be that much smaller and provide the same effective energy (power) output, and transformers (or small inductors) can be that much smaller operating at such high frequency (relative to your wall outlet's 50/60 Hz). The result is that the class D (switched-mode) power supply can be a tiny fraction of the size and weight of the old "llinear" supplies.
Thermal Management
Class A/AB amplifiers are at best around 50% to 70% efficient at maximum output, meaning 30 W to 50 W out of 100 W input is wasted as heat. That heat must be dissipated, and that requires large (heavy) heat sinks and perhaps active cooling (fan(s)). It is not coincidence that most A/AB amplifiers have a lot of open space inside, surrounded by great big heat sinks, to keep the amplifier operating at reasonable temperature.
Class D amplifiers can approach 90% or more efficiency at maximum output, so out of 100 W input only 10 W is wasted as heat. Heat sinks can be much smaller, to the point that many class D amps have small'ish internal heatsinks and the chassis provides the rest. That high efficiency also means that fewer active output devices can be used since they do not have to support high waste energy; they only dissipate significant power when switching state.
As an aside, there are a number of A/AB amplifiers that incorporate power supplies that can change their voltage on the fly in response to the signal. The power supply's voltage rails can switch among two or more voltages (class G) or actually track the input (class H), adjusting the voltage up and down with the audio signal. That way, waste power is reduced since the voltage is reduced when higher power is not needed, and similarly smaller heat sinks can be used since the average power dissipated is much less. Benchmark and other amplifiers do this.
HTH - Don
p.s. Here is an article describing basic class D amplifier operation:
https://www.audiosciencereview.com/forum/index.php?threads/class-d-amplifiers-101.7355/
