It's a single-ended triode (SET) amp, with no feedback, so it does have a bit of distortion - mostly second harmonic. It is more audible with low frequency sine waves because the ear's sensitivity increases rapidly with frequency in that range, and the distortion harmonics are higher frequency - mostly twice the fundamental. It is less audible with the Crack because the Mainline signal voltage at the tube output is much greater due to the output transformer.
A number of artificial signals have been documented in the literature which bring out low-level distortions like this. These small distortions are usually not audible with music, which is almost never a sine wave, and almost never a single, unvarying tone. SETs have many audible virtues, but low measured distortion is not one of them.
