If the engineer's goal is to have as little distortion as possible, then the amplifier or loudspeaker with the lowest number wins. Right?
Wrong. Just ask the receiver manufacturers who boast of vanishingly low THD but aren't worth the metal they're made from when it comes to high-performance audio.
Designing audio equipment that sounds good is an art. A balancing act.
The tools we have at our disposal to affect THD are many, but not all of them are something we want to use.
For example, negative feedback. Judiciously applied it's good. Overused it makes for hard and bright sound.
An input stage without any negative feedback might produce a relatively high THD level of 0.1% - but sound better than one with lots of feedback measuring 0.001%
Today's takeaway: it isn't the measurement that matters. It's how you achieved it.