By far the main source of degradation in any typical analog audio path is going to be transducers (microphones, speakers, phono cartridges, tape heads) and inferior media (tape, etc.). The vast majority of modern amplifiers, and high end older amplifiers, are extremely transparent with good margins beyond typical human hearing; any issues like noise, harmonic distortion, uneven response, inadequate damping factor, etc. introduced at the amplifier would typically be masked by quirks of the speakers, and revealed only by measuring upstream of the speakers.
