I've had about an hour listening to the H6, here are some initial impressions just after testing some of my usual demo songs.
The sound overall is very balanced, with a nice heft to the low end, they remind me of the Momentum 2 in that there is a fullness to the sound, but the treble is much more defined. They have better impact and bass quantity than the AKG K553, with maybe slightly less sparkle up top (not a bad thing at all for me). They remind me of a better version of the Sony MDR-1A, the Sony has a nice full sound but they are more veiled. The sound stage of these is pretty good for a closed can, the Sony's had a small closed in sound, whereas these are more akin to the AKG K553.
I am definitely liking these a lot, the ear cups are not the roomiest, but they are a comfortable headphone being so light. I have already ordered a new cable though, the one is comes with is junk and if I keep these it will get thrown away. I also want to try them with an amplifier, they get loud enough for my taste through a HiFiMeDIY Sabre USB DAC, but I am just curious if I can hear any difference given their high impedance.