The first beta version I heard had good bass and midrange, but a very atenuated treble. It sounded like the audiophile concept of "warm". I liked those for vocals, but for orchestral music, they seemed muffled. The second beta I heard fixed the problem and added in the high end well with just a little bump up of 1 or 2dB at 3kHz. The third beta I ran test tones on... it had 3-4 dB too much at 3kHz and 6kHz. It was just a tiny bit too bright, but could be easily EQed to correct. The production copy I have now is absolutely perfect, with a remarkably balanced response across the full range from around 28Hz to the upper limits of human hearing.
I'm betting you heard the first beta. That was the one mentioned in Jude's first report months back. Since then, the response has been fine tuned considerably. I wouldn't judge these cans based on the beta versions. There was some parallel parking going on in the response of the various versions.
The designer, Igor told us in the beta group that his goal for the production copies was a response that didn't deviate more than 1dB. Most headphone brands allow a 3dB drift from copy to copy. I'm guessing that Oppo's tolerances are extremely tight, because the retail version I'm listening to now totally nails the perfect frequency response. Some people might prefer more colored sound; but starting with a completely even response is fantastic, because it means that these headphones can be EQed into sounding any way a person might want.
These headphones are exactly what I've been looking for since I started out as a hifi nut... a balanced response where no frequencies jump out in front and block other frequencies. The complete lack of frequency masking makes these the most transparent and clear headphones I've ever heard.