Plse tell me more about the "others". The reason "total distortion" (not necessarily harmonic) is so easy to measure is that all you have to do is run a pure sine wave thru the amp, null it out, and see what's left. Anything left is by definition distortion - hum, noise, or whatever. I agree that there are lots of differences between amps, but let's face it: they are the least distorting part of the reproducing chain - unless you count the wires, which many of your contributors find make "huge" differences. Anyway, compared to electromechanical devices, amps can be "perfect".