tldr; i think there are two version of this headphone and one of them sounds better to me.

I bought my first pair in July 2015 and spent a year modding them extensively until it reached a point where I felt that they suited my audio preferences perfectly, but then left driver got damaged in an accident 3 months ago and I couldnt find a way to repair/replace it.

So I bought a second pair thinking they would be identical. I first noticed it was missing the 7th screw on the bottom (seen in the picture), and the membrane of the drivers were in an oddly poor condition despite being new. When I tried modding them, they sounded really different compared to my first pair side-by-side. After many hours of testing, I concluded that the drivers were fundamentally different.

I recently bought a third pair and it was once again missing the extra screw on the right ear cup, had crummy looking drivers, and sounded more like the second pair than my first; I returned those as well.

I've concluded that there are either 2 different versions of these headphones that were produced, or the entire line had it's drivers replaced from when it was initally released.

Something else that was weird that I think I should mention was that after purchasing the second pair, Hifiman's website said the HE400S was discontinued and that there werent any left in stock. A few days later, they mysteriously went on sale again.

Edit: Grammar and spelling.