Not sure where you're getting your info, Gonk, but Class-D amplifiers are switching mode amplifiers that operate on a PWM signal. A digital input (like SPDIF or HDMI audio) can be directly converted to PWM in a DSP chip without ever going into the analog domain. In fact, HDMI supports sending multi-channel PWM (a la DSD, as is used with SACDs) directly.

A pre-pro internal digital signal that is converted to an analog output and fed to an analog input on a Class D amplifier must still be converted to PWM inside the amp. This adds an unnecessary D/A conversion at the output of the preamp/processor, and also requires the signal to travel in the analog domain through conventional analog cables between the two boxes- things that could be avoided if the signal was left in the digital domain all the way to the DSP in the amp.