Sound that passed through a Vacuum
We think the reason why vacuum tube is superior in sound quality to semiconductor is the difference between energy passing through solid and energy passing through vacuum. Semiconductors emit white noise as energy is released as heat that electrons and holes will collide with particles as they move. Since the vacuum tube has no resistance in the vacuum state, there is no collision energy, white noise does not occur, energy transfer speed is very fast.
The sound quality of an op amp is often the crucial selection criteria even when a data sheet claims exceptional distortion performance. By its nature, sound quality is subjective. Furthermore, results of listening tests can vary depending on application and circuit configuration. Even experienced listeners in controlled tests often reach different conclusions. Many audio experts believe that the sound quality of a high performance FET op amp is superior to that of bipolar op amps. A possible reason for this is that bipolar designs generate greater odd-order harmonics than FETs. To the human ear, odd-order harmonics have long been identified as sounding more unpleasant than even-order harmonics. FETs, like vacuum tubes, have a square-law I-V transfer function which is more linear than the exponential transfer function of a bipolar transistor. As a direct result of this square-law characteristic, FETs produce predominantly even-order harmonics. Figure shows the transfer function of a bipolar transistor and FET. Fourier transformation of both transfer functions reveals the lower odd-order harmonics of the FET amplifier stage.
The reason for the high quality sound, even though the tube amps have a bit higher distortion ratio than the semiconductor amps, is due to the difference in each harmonic order. According to the studies done by scientists, a person's ears showed 'Even-order harmonics' rather than 'Odd-order harmonics' that occur in the tube amps. So tube amps distortion ratio is a bit high, but they can be heard harmonically or musically, but semiconductor amps distortion ratio is low, therefore they sound 'frozen'.
Unique Sensibility of Tubes
In these days of competitive state-of-the art electronics that are manufactured with semiconductors, you may think that the tubes are regarded obsolete and outdated antiques that go against the current of the times. But the reason why the tubes are loved over the time and still regarded the best sounding by many audio enthusiasts is that the tubes ability to produce rich sound that is musically enriching. And this cannot be achieved with digital sensibility.
This is a partial citation from Burr Brown's article.