A low power class A amplifier sounds loud enough at it sounds beautiful

 At your risk.

There are at least 2 other reasons.

Steven's law. How ear brain perceives sound intensity (ie power). 10 times less power sounds only 2 times less loud. So for example 100mW should sound only 4 times less than 10W. See Sound and Hearing - Stevens, Warshofsky - Life Science Library.

Nice distortion at peaks simulating real life instruments getting gradually non linear at high playing levels. See the paper Tubes vs Transistors - Russell O. Hamm.