Interesting Nelson Pass project and the philosophy behind it. Nelson writes: lWhy do we care about 2nd harmonic? Historically, audio amplification has been about the elimination of all forms of distortion. Early on, the premise has been that low distortion is one of the keys to audio quality and this was certainly true back when amplifiers had a lot of distortion. But in the past forty years or so distortion has been reduced to very small amounts, and as a practical matter the problem is solved.”
“However, as Marshal McLuhan noted long ago, we turn our mature technologies into Art and Entertainment, and many audiophiles want something different – subjectively good sound.
Looking back to the glory days of tube sound, some speculate that the 2nd harmonic character of single-ended tube amplifier circuits is an important element of the sound quality they want. A common complaint is that modern recordings and reproduction are “clinical” and lacking in “organic” qualities. In the musical instrument and even the recording industry doses of 2nd harmonic distortion are routinely added to help create “the sound”.
This is a factor in the popularity of single-ended tube amplifiers in high-end audio systems. Even owners of technically accurate audio power amplifiers occasionally choose tube pre- amplifiers to “warm up” the sound.
Some purists argue that this is some sort of violation of the recording, that playback should be as accurate as possible. It’s a valid opinion, but it’s not my purpose to argue that point here.
My own view is that this is entertainment – not Dialysis – and the customer should use whatever makes him happy. After all, he is the guy who pays for it.”

Complete article: link.