gonk & barend are correct. Class G is not technically switching, although it does "switch" power rails.
A true switching power supply actually breaks up the input power fromthe wall and switches it at a high frequency to adjust to the power demands at the output. This switching causes lots of noise, electrical, RF and in some cases mechanical. The filtering is critical to not having this noise get through to the actual audio signal.

Class D is a bit different in that it is switching the input audio signal, and adjusting the output based on the input demands. I have heard class D, but have not done any critical listening or measuring to see whether it has a significant noise contribution.
By far as others have said, with the exception of class D, a clean DC power is what all amplifiers need for the output circuits.