I have now had several hours of listening on my HP50s and feel confident in the limited impressions I will share. I do not really believe in burn in periods, or not enough so that I feel any audible change can be accounted for due to "burn in" so limited hours will still work for me. First I will say there is no issue with rolled off treble, not to my ears anyway and I am going on 46 so that part of the frequency range should be diminished for me meaning if the HP50 was deficient I would notice it. I owned an HD650 and I noticed the slight, but perceptible veiling, and it was a newer driver HD650 so we can put to bed whether or not I can detect rolled off treble to some extent anyway. I find the treble excellent with the HP50, bang on for me in fact. It is quite present, but never strident or edgy. Plenty of upper frequency energy here.
My overall impression is of clarity of sound and balance. Up and down the frequency response this headphone sounds poised and detailed, but all of the elements are presented in a balanced fashion. There is plenty of bass and I mean plenty. I owned a set of D7000 which of course produce tons of bass and the HP50s get that significant pressure going for me. I listen to some pretty low frequency heavy music such as Deadmouse5, Thievery Corporation, Tool, Bluetech, and Phutureprimitive to name a few tested to this point and nobody should fear these are bass shy, unless you are a serious bass head, which thankfully I am not. Sorry, never understood the desire to have one particular element of sound dominate and cover everything else up, makes no sense to me, but what do I know?
I find the midrange to be quite nice with vocals more forward than with my current portable cans (V Moda M80). Room Feel as NAD calls it is very nice, and I would have a difficult time explaining the psychoacoustics of it so I hope somebody else more skilled in this area has done so, or will do so. Suffice to say that there is a depth of sound that is quite pleasing and makes the soundstage seem more three dimensional than my last few portables (DT770, HF2, M-80). The bass is slightly less round perhaps than with the M-80, which is another sound signature I quite like, or perhaps it is simply that the bass feels bigger due to the larger driver of the HP50 which may account for this. That would be the only aspect that I could even suggest is not extremely enjoyable with the HP50, that the bass could be a little rounder and wetter sounding. I realise such descriptors don't have a great deal of utility as they may mean nothing to other people so I would be curious to see what others feel about this. That said, I really, really do like the low frequency capabilities of the HP50.