FYI, I asked Jerry about "burn-in" and he agreed that BA drivers do need some use to loosen up. With my JH-13s they started out sounding completely off, with almost no bass to speak of. After some hours of use, the bass appeared as if another driver had been added. This was not my brain adapting to the sound, as I wasn't using them daily. This doesn't, of course, mean that this will be true for every pair of headphones or IEMs.
Anyhow, as for the "dark" impressions, it isn't that I felt they had a rolled-off treble, but more the sense of the music coming out of blackness. I associate this with a lack of ringing or other types of distortion that are so common in headphones that we get used to them as being normal. I did have some discussion with Jerry in which he talked a bit about the issues with treble quality in IEM designs that he had worked to overcome, so I'm guessing it is somewhat related.
I remember my first impressions of them that they had an un-IEM-like sound. My JH-13s still very much sound like IEMs to me. The universal loaners should arrive today or tomorrow though so I'll focus on them for a week and see what I can figure out.