I haven't tried the Pro900 (yet) but based on what the other guys have said here, I am anxious to try them based on how much I already love my ES10s. I doubt that you would regret the purchase and there should be many who will want to buy them from you if you decide that they are not for you.
Out of curiosity, what is the pricing of the ES10 in Japan? And is it any different than those sold in North America?
Good luck and be sure to let us know what you think since you are a much more qualified evaluator than I. My listening and commentary skills with respect to HPs is similar to shamrock's post #6 above.