This confirms what I've been saying, and also that frequency response is probably the most important aspect of (perceived) sound quality.
Okay, headphones are not perfectly minimum phase, but what is perfect in reality. After all, most headphones do not have any damping material at all. It's basically just a driver screwed into a plastic cup, so resonances are a given. But with some modding and EQing you can achieve pretty much any sound you want.