Archimago has posted a little master class in how to evaluate amplifiers:
Among the descriptions of what he’s testing, he proposes a new “scoring system” for amplifiers which consists of three measurements: the damping factor (a measure of how well it can drive low-impedance or difficult speakers), what he calls the Triple-Tone Distortion Factor (basically THD at low power), and the Power Factor (how much it can produce without too much distortion). Then he proposes what some typical scores might be for various typical amplification systems:
UltraAudio Linear Amp = 120x / -90dB / 21.5V
SuperFi HyperTube = 15x / -45dB / Insufficient
FleaWatt Amp = 25x / -74dB / 3.2V
ShoutOut MegaPower PA = 80x / -53dB / 33.5V
GuruMeditation Ideal Wire 250W Amp = 300x / -100dB / 31.5V
Of course, his “goal” in an amp is transparency, not decorative coloration – not everyone shares that goal.