Two different amp models with the exact same THD figure can sound very different. The audible difference is a result of the distortion's harmonic content. You can listen to a tube amp that has 1% THD and it will sound better than a solid state amp with the same THD. The solid state amps tend to generate high order odd harmonics while the tube amps generate low order odd harmonics that are less offensive to our ears. But what about solid state amps that have a very low THD figure? Does harmonic content really matter? Are the differences that KlipschGuy (BTW, ditch that name) hears the result of the specific amplifier switching topology?
JohnK is right about frequency response. For practical listening applications, modern solid state amplifiers reproduce every frequency in the audible range with equal emphasis.
Maybe the differences that we hear are due to differing slew rates or output impedances or clipping distortion or open loop responses or speaker to amplifier interfaces. Or maybe all of the above combined. Who knows? The fact that there are audible differences between amps is actually obvious to most. The degree of difference and why there are differences is not.
