Amplifiers?

The "issue" is that the differences in on-linearities ("color") are also a function of volume. While this is a very subjective matter, it is quite real. As the sound enginer for a band, I did a number of tests on the frequency analysis (including phase, attack, distortion, etc) on a number of performances. Given the same material being played at different venues, the profiles were quite distinct. In recent years, a number of audio equipment manufacturers have released DSP based products that mimic the behaviour of specific tube based amplifiers. While some purists disagree, the majority of blind sound tests have confirmed that these are indistinguasible from the originals to the human (trained) ear. So a "perfect" amplifier is not really what is desired in most conditions.