I have both and appreciate both the ESW9 and the ESW10JPNs. The ESW9 were a surprise to me and shows that Audio-Technica is definitely developing new methods for sound presentation. The ESW10JPN takes this much further up the ladder. I think that AT and Ultrasone are two headphone manufacturers that are working hard to move the quality bar beyond the status quo that exist today in the single ended arena. DIYer's have move the bar with other manufacturers equipment using different methods but it seems that most manufacturers are satisfied with the sound quality achieved or don't even think it necessary to keep what they have.
The ESW9 is more mellow, warmer all across the sound frequency. The ESW10JPNs are much more detailed and one word that comes to me is "pure" compared with the ESW9. They are "very Electrostatic" sounding in comparison, fast, precise and evolving. I thought the ESW9 was musical until I got the ESW10JPN.
The ESW9 is musical but not in the same way and does not present the music with the same clarity. Another thing about the ESW9, I have owned two of them. My first one was very early in their production run. I thought it's bass was muddy on fast music but the second one, a much later production run, is much better in this respect. It still has all the other sound qualities of the early ESW9 but it's bass is simply better.
The bass of the ESW10JPN is faster, tighter and just as powerful. The mids of the ESW10JPN are better as well. The only issue that I have found with the ESW10JPN is that with some tracks the high frequencies are just a little to pronounced. Using it with my tube amp maximizes the ESW10JPN sound much more than when the ESW9 is plugged in to it and also minimizes the issue that I noted before.
To me the ESW10JPN is a player in the high end of single ended dynamic headphones and can stand up to almost any other manufacturer's dynamic headphone in production today.