That's from the old "crossover noise/distortion in transistor amps" thing from ~50 years ago. The earlier solid state amps were easy to distinguish from the tube amps of the day by the high frequency distortion present (assuming one's hearing was sufficiently acute in the higher frequencies). Newer types of devices and different circuit designs fixed that long ago, but the idea keeps getting resurrected from time to time, probably by makers of retro tube amps.
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