Bwoca,
Is there an paper at
http://www.intersil.com/, about the ic HIP4080, with some theory about class D amps. It's about efficiency, distortion etc.
For low frequency class D amps are a very good choice. You can achieve the high power levels neede at these frequencies with high efficiency and little heatsinks.
For the rest of the audio band, since you don't need big power to achieve high sound pressure levels, they are not the best choice, because THD in this class of amp increases if you don't use high switching frequencies. Typically you need 20 times (or more) the higher audio frequency that you are working on the switching circuit to achieve low THD levels.
Regards