It can be objectively measured. Tubes tend to produce even harmonics, which are pleasing to the human ear. The even harmonics sound like octaves of the origin

It can be objectively measured. Tubes tend to produce even harmonics, which are pleasing to the human ear. The even harmonics sound like octaves of the original pitch, creating a rich sound. Solid state tends towards a mix of even and odd harmonics. The odd harmonics are not pleasing to the human ear because they produce pitches not easily resolved to the intended pitch. Ironically, solid state amps can have a lower total harmonic distortion than tubes but produce a sound less desirable to humans (in general).