What about stability at louder levels? Possibly, one driver may not do so well when you push it harder. If you look at distortion graphs, headphones puts out more distortion if you drive it harder. Yeah, I sold my ER4. My ears don't lie, I didn't like them(I could care less how accurate people think it is, it isn't to me, I personally don't think it's neutral, but really cold and unnatural sounding). But, expectation bias maybe from looking at the graph and wanting it to be accurate. Newer graphing methods are available now, than what Etymotic has used in the past.