Analogue tone controls must have both a resistance and a reactance (capacitance or inductance). Both of these are imperfect devices unless you spend very big bucks (think radio station and vacuum capacitors). Unless your tone control costs six figures, it must introduce distortion.
Digital has its own problems, but I don't use that technology (don't like it), so I can't give you chapter and verse.
Digital has its own problems, but I don't use that technology (don't like it), so I can't give you chapter and verse.