Valve And Transistor Audio Amplifiers -

High; energy-efficient, runs cool, and is immediately usable.

: Distorts gradually and smoothly, creating a pleasing compression.

Compact and lightweight; can be miniaturized into tiny devices. Valve vs Solid State Amplifiers - Audio Affair Blog

: Abrupt distortion that can sound harsh or "buzzy" when pushed. Efficiency Low; wastes ~75% of energy as heat; requires warm-up time.