Geranium is the plant, germanium is the transistor. But everything is silicon, and has improved for the usual Moore's law reasons.

A class D amp is a closed-loop control system. While it can be done entirely in analogue, having a microprocessor in there lets the designer algorithmically compensate for nonlinearity of the output.

Most commercial switchmode amps are fully analog. The large power amps with built in DSP tend to process the audio signal before it reaches the power amp circuit. Artifacts such as harmonics can be driven down way below any credible threshold of audibility, with conventional techniques.

Especially in big amps, the power amp already has some rather heavy responsibilities, just to be stable when driving odd loads, and generally not blowing up. This tends to discourage putting a lot of weird stuff inside the feedback loop.