It has been a while since I read up on class D, so I had to go back and do some digging. I have read in several places that the common term "digital amps" is a misnomer, with the "D" simply being the next letter in line at the time that the technology first appeared. (See this Wikipedia entry, which mentions both the class D identifier and touches on PWM as pertains to analog signals.) Another article that touches on digital vs. analog input to class D amps is this Audioholics article. It does point out that it is technically possible to build a class D amp that accepts a digital signal (which I had either not heard before or had forgotten), but it goes on to suggest that this approach may actually be inferior to a design that operates with an analog signal.