Just back from the NYC event, finally heard the re600. I briefly compared them with my re400 and JH13 (well, I know, this was not fair...) anyway..... my impressions are by no mean comprehensive due to the event situation and short listening time, but I am doing my best.
re600 sounds really nice, but not enough for me to swipe my credit card for now, here's why:
hpp1 -> re600 / re400:
they are very similar in sound signature, balanced through out the range. To my ears, re600 have slightly wider and deeper sound stage, a more details and extended treble but not too much(re400s are already excellent), the bass seems to have more body and flesh, again, not much, but noticeable if you hear carefully. Overall, the difference is there but not that big
hpp1 -> re600 / JH13:
JH13s have deeper sound stage and instrument separation, I could not hear clear difference in the mids of those two, but the highs and bass are different. JH13s have more bass in quantity, but I felt re600's bass is slightly tighter. they are both very fast. And re600 have well extended highs, even further than JH13 to my ear, but in comparison, I like JH13s more because the highs from re600 are sometimes too sharp for me.
hm901 -> re400(SE) / re600(Balanced):
here is where re600 beats re400 in a wide margin, it is a whole different animal in the balanced mode. Extremely rich in details and super transparent on the balanced re600s, the vocal is more colorful. Sorry for not being able to say more bout the balanced re600s on 901, only listened to 1 song and did not have chance to plug the JHs in 901 due to the angry waiting people in the line
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Also, the hpp1 can not drive HE300/400/500 to the same league as re600s, this little baby sounds way better out of my hpp1.
I finally decided to stay with my current rig and save the money for hm901+balanced card since I do not have enough fund for the 901 combo for now, but I will seriously consider the hm901 with the balanced amp card and re-cable my JH13s in a few months.