Say Goodbye to the Last Vacuum Tube Product

Absolutely not impossible. If you can define it you can design it.

Class A uses tubes, and works in the simplest and most accurate way by amplifying the complete signal. It maintains the most important characteristic needed to perfection - linearity of waveform amplification. When you try and get extra juice out of it, instead of the annoying clipping you get from digital amplification, you get a nicer sounding and more gradual reaction. The main drawback is energy efficiency. There's a theoretical peak efficiency of 50% to Class A amps, and what's worse is that it's usually less than half that. It makes tube amps that aren't small require massive heat sinks, and increases the BOM.

Class D is totally different. It dumps linearity out the window in favor of energy efficiency. It basically outputs a bunch of pulses, and then applies a low-pass filter to remove the annoying artifacts. While you might say that it's close enough - it isn't for high fidelity systems, especially in production studios, live performance, and audiophile cases. While competitive in quality with the low end of amplifiers (under $200), if you are looking for accurate detail reproduction there is no replacing linear amplification yet. The advantage is that even at the worst use scenarios, Class D amps have over 50% energy efficiency, and usually manage over 90% for higher volumes, making them need far less in the way of heat sinks which allows for leaner designs, and a better price/performance ratio.

The Class D amps will eventually win out. Class A amps have decades of R&D as an advantage, but Class D is improving by leaps and bounds. Eventually, Class D will become "good enough" and replace Class A altogether because of the advantages in price and energy efficiency. But that day has not yet come.

That being said, I use a class D amp for my home set up. It's close enough, and my power bill thanks me monthly.

But I only have 22 years experience playing guitar and bass across genres as diverse as jazz, blues, gospel, metal, modern rock, and folk, so hey what do I know?