Probably due to the 10k spike you are hearing :p The treble does downslope though a bit in the upper treble.
Bad fit on the SE535 and your UE900 is stock. UE900 is not a very good IEM stock, but it's also one of the best IEMs when changed a bit. Very strange case, I think the pinhole and low impedance were last minute changes to please the bassheads and volume likers.....S535 has rolled off treble IME, only slightly better than 530, UE900 has one of the most extended trebles of all IEMs........ even stock the UE900s treble is anything but rolled off, not even close, the lower midhighs are conservative though.
Don't take my word as I am going only by my ears. Others like Inks have their ears as well as the charts sine-waves and decombopulator rods with the flux capacitors to prove I am wrong.
I certainly prefer the TF10 over the UE900. Both have flaws but I could live with TF10's moreso. At least the bass and treble extend further. Midrange is the only real issue with TF10.
I bought and sold my UE900 last week. I will have another pair of SE535's tomorrow (owned it 6 months just after released) and will post a direct comparison as close as I can from recent memory of having the UE900.
Just like me! I own both and find UE900 much better overall than SE535, altough lacking some bass. My ears simply cannot stand the absence of treble coming from the 535.
My 535 is new (regular version, less than 50 hours) and I'm about to sell it in the coming weeks if the highs doesn't improve.
Perhaps the problem is the veil? Or my interpretation of what a high extended treble should sound like (bright and crisp). I guess the extension could be there...and be higher but in this regard I would always expect something more bright and crisp if it is more extended. SE535 is brighter and crisper to my ears. UE900 and W4 are not bright and crisp because of the veil and is why they both sound a bit "off" to me. Maybe it is an unveiled midrange with SE535 making it sound like it has more clarity overall which I think it does.