Recently I noticed that THD property of IEMs has no relevance for their price range.
Moondrop Kato for example has a THD of <0.15%, while the more expensive Moondrop Variations is <1%. Does that means that the cheaper Kato has 6.6x less distortion than the Variations?
I am starting to think that multi-drivers IEMs have more distortion than single dynamic. For example, Sennheiser IE200 has a THD of only <0.08 % - all recent Sennheiser IEMs have very low THD, they are all single dynamic. Is that why Sennheiser never use mutli-drivers or Planar? Thieaudio makes expensive multi-drivers IEM - suspiciously they never published the THD of their IEMs.
Planar is known for low distortion - IEM like 7hz Timeless has a THD of <0.2% but still can’t compete with Sennheiser’s dynamic IEMs.
The THD is usually measured at 100db, very few people listen to music at that volume level. However when the music swells in a crescendo section, with many instruments and vocals at full blast - this is where distortion happens. These ’climatic’ section are the best segment to test IEMs/Headphones. Lesser listening device will sounds harsh and messy, better ones will retained their clarity and resolution.
THD, no, it's worthless. The individual orders as a graph, not a single number, yes, that's very useful. Granted it's probably not that useful to you, but it's invaluable to engineers designing a headphone.