Final review
Sound
Quite simply, superb. The detail and resolution is the best that I have heard from any headphone. It is like being moved closer to the music. Bass is there, but like AKGs in general and the K702 in specific, it is bass detail not BASS thump that comes out of these headphones. They can do the intro bass on Massive Attack's 'Angel' with only minor distortion. But they lose some of the energy and power of the track. The treble is beautifully detailed without sharpness or sibilance as tried with New Order's 'Regret'. My tester track for how well something copes with a badly recorded track is Ash's 'Oh Yeah', a flat and lifeless recording. It sounded great and produced life and vibrancy that I have not heard for a long long time. Gomez's 'Whippin Picidilly' is the soundstage and detail test. All detail was there but the soundstage is not great. But the sheer size of the headphones around your ears makes for a big sound, all be it inside your head and the headphones themselves.
They work with all the music that I listen to, including metal, though to those who prefer or are used to bass attack, thye will initially lack dynamics.
Build
They come in a lovely box and with a long cable that can be adapted to be used off a jack or phonos to be plugged into a speaker amp. They are very difficult to drive, the 1.3 W output of a Musical Fidelity X-CANV8P struggled and I found clear listening was at too high a volume for safe extended listening sessions. They leak sound like a speaker leaks sound!
I found the soft pads dug in a bit to my jaw and they clamp well onto your head. The headband is lovely and soft and support what are heavy headphones very well.
Overall
They are an upgrade from the AKG K702s and fans of that headphone will love the HE-6. But you need a powerful amp, relatively speaking for headphones to get the best out of them.