A preamplifier might load the microphone with low impedance, forcing the microphone to work harder and so change its tone quality. A preamplifier might add coloration by adding a different characteristic than the audio mixer's built-in preamplifiers.
A preamplifier may burden the microphone with low impedance, causing it to exert greater effort and thus altering its tone quality. A preamplifier could also introduce coloration by adding features not present in the audio mixer's preamplifiers.