The “D” does not stand for digital. It was simply the next available letter after naming class A, B, and C amplification technology. It is true that a digital kind of class D exists, usually employed in PA devices and cheaper models. However, what the designs of Custom D-Class amplifiers employ is the analogue kind – and king – of class D; these are indeed 100% analogue amplifiers.
Class D theory goes back to the 50’s. It took until the turn of the 21st century to successfully produce good sounding class D amplifiers.
The advantages are many; the most prominent being a small footprint, low heat emission, and high efficiency. For example, our Dual Mono amp draws a mere 35 W under load to deliver 2x 900W~4ohm, whereas a comparably powerful A/B-class amp draws 10 ~ 50 x that amount.
In terms of sound, the very way class D works, translates into a neutral sound. If well engineered, the design will be musical and warm sounding to boot, whilst never deviating from that detailed, hi-resolution quality.
For further reading here‘s an excellent article written on the workings of class D amplification. Many more writings can be found through your search engine of choice.