Very interesting thanks, what I wanted to know is not so much about the FR, but whether the 58x is technically as good as the HD660s, for example people hearing "more grain" and "transparency" might be a) the frequency response is different b) the driver in the 58x is worse. If it is "a" eg. just the FR causing any differences, then that is not a problem, because you can either keep the FR the same and have a 660s with more bass and a bit more treble, or you can mod them and have very close to a 660s. BUT if the answer is "b" and the differences are due to worse drivers, then the 660s would possibly still be the better option. But from what you are saying there and what I have read in some other places, it looks like the 58x could be the same as the 660s but with different tuning, I actually think the 58x might be better, because it has more sub bass, which is my main problem with the HD6xx line and most open headphones. Very tempted to give the 58x a try because they are so ridiculously cheap if they are actually the same as the HD660s.