BTW if you want to see low THD, the AirPods Max is a good contender : https://www.head-fi.org/threads/apple-airpods-max-measurements-brüel-kjær-5128.951184/
Apparently the Airpods are also very well tuned and the tuning of the SR-009 has been criticised. So in response to the original question - what, objectively, makes the Stax "better" - I don't know. I haven't heard the Airpods.
Just waiting for the ASR review/ comparison to Stax headphones. If there doesn't appear to be any objective reason why the Stax is 'better' and if I like the sound, I might sell up, get the Airpods and pocket the price difference.