What are the advantages of class-A amplifiers?

Fidelity. The one overriding advantage of class-A is high fidelity.

Class-A delivers the lowest distortion and smoothest sound. That’s what class-A is designed for, and that’s why ALL the very best amplifiers are class-A, where cost is no object.

It also delivers disadvantages, such as the greatest heat and weight and the largest most expensive power supplies, heatsinks and chassis. Class-A amplifiers have the highest cost per Watt, so cheap consumer designs avoid the use of class-A.

For the best sound, whether using tubes or transistors, class-A is the gold standard.