Please help me understand

There are two concepts I've never been able to reconcile. How is it possible for different amps to have similar distortion levels (inaudible), yet still have what audiophiles might describe as a "tonal signature". In other words, how is it possible for an amp to have a perceived "warm" or "bright" sound, if it's accurately reproducing the input signal? It seems to me that all high quality, properly functioning amps should sound the same. If they don't, them some of them aren't doing their job very well.