61, of course you are right about an amplifier's being supposed to reproduce the input signal it's fed - in theory. But in the real world, no amplifier can do this perfectly. So amplifiers introduce colorations, have strengths and weaknesses, sound different from one another, and may have characteristically indentifiable traits within design types - meaning that certain design schools may tend to broadly share some relative strengths and weaknesses.
By the same token, different types of music will tend to differ in the characteristic demands they place on the equipment reproducing them. Tube gear is commonly considered to be capable, when well-executed, of standard-setting reproduction of pure harmonic overtones and sonic recreation of space and objects. These strengths correlate well with the demands generated by naturally recorded and live-captured acoustic music and voice. With electronically produced and studio recorded and processed multi-track music (like most rock), the demand for the above qualities can be lesser because little genuine space or acoustically natural harmonic overtone content may be present. The traditional solid state virtues (again, when well-executed) of separation, definition, and bass weight and slam can be of more relative importance here. Obviously, and as previously intimated, these are broad generalizations, and quality amps of all types can do a capable job with many sorts of music types. But differences will still exist, and an audiophile's preference in types of music listened to will often have an impact on which set of amp characteristics are weighted with the most importance for the job.
