There's a certain unavoidable amount of distortion in any audio circuit containing vacuum tubes; this takes the form of "soft clipping", which adds some harmonics and affects the volume dynamics a bit. Unless you deliberately overdrive the tube amp, this effect will be fairly subtle, creating the "warm" or "fuzzy" tone that people consider characteristic of tube circuitry.
Transistor circuits introduce a slightly different kind of distortion (different harmonics), and at much lower levels, making the music sound "sharp" (not as in "higher-pitched", but as in "the opposite of fuzzy") or "crisp". The very first time I heard it, I liked it better. (Nowadays, I recognize that both sounds have their uses.) There are even solid-state circuits that purport to mimic the old tube sound, but to my ears, it's still not quite the same.
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Transistor circuits introduce a slightly different kind of distortion (different harmonics), and at much lower levels, making the music sound "sharp" (not as in "higher-pitched", but as in "the opposite of fuzzy") or "crisp". The very first time I heard it, I liked it better. (Nowadays, I recognize that both sounds have their uses.) There are even solid-state circuits that purport to mimic the old tube sound, but to my ears, it's still not quite the same.