Hmm, I don't think it's an issue of my ES5 needing more burn-in to remove the veil, since I've been using them for several months. IMO, the ES5 is a great headphone, it's just warmer and has less instrument separation and clarity than the UERM.
I don't think the UERM's treble increased noticeably to me since I first got them. If anything, the UERM's treble presence and crispness were some of the things I was most impressed with in my initial listening sessions.
I haven't tried the AMB or Pico Slim (I plan to get a Slim eventually), but I have had the CLAS for a few months. Not sure if you have any specific questions about it, but if you're already heavily invested into compatible iDevices and either have a large library of ALAC or don't mind converting to that format, then for me it's almost a no-brainer. If not, then it seems there are several other very good portable sources, of which I've only heard the HM-801 and WhipMOD.
I'm still planning on writing a more detailed comparison of my customs one day, but for now I don't feel any particular urgency because I'm so satisfied with the presentation and detail retrieval ability of the UERM. For the first time in a long while, I've gone a few months without feeling a strong itch to demo or try another headphone. Even the JH3A's siren call hasn't been particularly seductive to me. How long will it last? Who knows, but to a compulsive upgrader like me, it seems to be a strong indication that the UERM is the custom I was looking for all along--and I would have never guessed it without hearing the demo.