I agree, after all the description of being dark I was floored when I noticed so much treble detail and neutrality in the mids. It's only half-way dark colored. I've seen the "u-shaped" description of few times, and I'm not sure I agree. The FR charts don't confirm that either. It seems like a bumped low-end, a neutral mid-range, a peak & flat plateu in the upper mids to keep them above the bass, the infamous 1kHz dip, and then mostly flat before rolling off. Effectively it's a very neutral Beyer/AKG like presentation above the bass, and a...something between Denon and maybe Sony bass end and a Sennheiser-like rolloff of he extreme highs and upper mids. Or as Fang refered to it, "American speakers like JBL." Despite the bashing Harman/JBL often gets, JBLs voicing is still oh so cool, and most venues I've seen amplified live performances in were using JBL, so I've never had a problem with sticking to that tonality. It's a very addictive blend of dark and exciting. Usually dark is paired with laid back and exciting with bright.
I think of them as HD650's younger, more athletically gifted adolescent brother.