Where was I? Oh yeah, man-made. There is one thing man has made that breaks this rule. And its not tubes, wire, speaker cones or cabinets, and it certainly isn't records. Its CDs. And yes, digital distortion levels are lower. Absolutely. But we're not talking absolute. We're talking odd relative to even. With digital they are all very low, meaning the irritating odd are just as loud in level - which means much worse in effect. With analog our beloved natural even-order harmonics overwhelms the man-made odd-order noise.Its not odd orders that make CDs not sound right. Its **inharmonic** information, which the ear/brain system interprets about the same as odd orders in that it is interpreted as harshness and brightness.
Inharmonic distortion is intermodulations where harmonics are created that are not related to the fundamental tone but the difference between the fundamental and something else: in digital- the scan frequency. It sounds like 'birdies'- little squeaks and squeals. Normally you don't hear it as it happens too quickly; its more audible if you take an analog sweep generator and record that and then play it back. So inharmonic distortion is a form of IMD which is to say its really audible, but its rarely measured as there is a simple way to get around it- record the sweep tone using a digital means rather than an analog sweep generator and presto! no inharmonics.
So its not a spec that shows up. But its a thing that analog simply does not have unless its really broken (you can get it if the record oscillator on a tape machine is intermodulating with harmonics of the signal being recorded but this only happens if the incoming signal is overloading and the record oscillator frequency is too low).
Aliasing is also a form of distortion but the digital world doesn't like to express that as distortion. But that's totally what it is and it too is really audible.
If/when digital eliminates these problems then it will sound like analog. Its certainly gotten a lot better over the years but digital began in the early 1980s and we're still dealing with it, so don't hold your breath...