Now there are two components at least in the equation which will influence ones decisions / conclusions, because different amps may (and probably will, because there is no "perfect" amp really) couple differently to the very same (complex impedance / phase etc.) speaker system, thus introducing all sorts of "new" types of distortion into the listening experience. This is very well known and (mostly) accepted for tube amps already but mostly ignored for solid state type amp designs.
Thus the simple combination of different amps, esoteric cable designs and a specific speaker system may already cause (subtle?) audible differences in sound, which are not attributable to the amp itself but just to the combination and complex interaction of amp and speaker.
Note, that the measurements provided by the amp manufacturer are usually done with a simple resistive load, thus eliminating this specific influence and its effects completely from the measurements provided.
The same thing but to a much lesser extent may happen to the combination of signal source and amp, which might take its toll also, but as stated, to a much lesser degree and may not even be audible any more by the human ear as long as output and input impedance are not crossly mismatched.
I would characterize this type of sound alteration as a sort of distortion too.
Edited by gurkey - 11/16/12 at 8:27am