The 'digital' nomenclature in Class D amps does not talk about the input or output but how this topology makes the transistors switch on and off very fast (PWM or some other types of modulation). The efficiency in Class D amps came from the fact that transistors don't waste much power when they're either passing current or blocking them (switched on or off). Transistors becomes lossy and lose efficiency if it is only partially passing/blocking signals (ie. Class A, AB amps).

Class D amps are ok typically for subs. Purists would still try to avoid them for any other purpose. Also, not all amps are the same. Some Class D amps lose more sound detail than others. Though, the filtering technology have vastly improved and is still improving.