That's a wise warning. Quick-swapping very similar entities like 009 and 009S can lead to erroneous conclusions. The ear/brain system can adapt to a lot, and your perception of the sound (how good it makes you feel to listen to it) can change significantly after the adaptation period. Or rather, some headphones have flaws or quirks that initially make you reject them, but can be adapted to, and after the adaptation period you fall in love. The most egregious example of this, for me, was the Sony Qualia 010. Another problem with quick-swapping is the brain's tendency to pattern-match signals, which can have the exact opposite effect - this can obscure certain subtle differences or quirks/flaws that would disturb you in long-term listening. This is also why I feel that DBT is generally crap for high-end audio. Fortunately both the 009 and the 009S fare extremely well in the long-term listening satisfaction test! Especially once you have an amp at the level of BHSE.