Since I'm not a verified owner here because I don't live in the USA, so I had to use a freight forwarder to buy for me, having said that, my first impression coming from the Denon AH-D2000 and AH-D5000 is, the hd6xx is very loud if an amplifier is used, as the Denons are 25ohm, I myself got used to an amazing clear and yet not loud sound on the Denons I mentioned.
Regarding impedance, note that the hd6xx is 300ohm, so, on my tests, at 0ohm to 64ohm the hd6xx lacks detail and everything else that the Denons have to spare as the hd6xx was not meant to be driven around this level, at 64 ohm to 300ohm is what the hd6xx really starts being detailed but as far as I can tell, it is unbearable to listen to on volume 100, you will get deaf if you do, to able to listen to this without getting deaf I had to reduce 30db on foobar 2000, at 300ohm to 600ohm is a just a tad higher but the loudness is not as unbearable as from 64ohm to 300ohm, I would recommend to listen to the hd6xx at 300ohm to 600ohm because that is where is very detailed with a rich sound all around and do not forget to reduce the volume, at 0ohm to 64ohm it does not need to reduce the volume, from 64ohm to 300ohm the volume needs at least 30db to be reduced, from 300ohm to 600ohm 35db.
The first 30 minutes listening to it gave me a terrible headache, they are too loud on volume at 100, and even if you listen to it for few seconds, it makes your ears to bleed. I used it always on equalizer off, hi-fi mode, so nothing to contradict my ears.
For now lets say that the Denon AH-D5000 is better than the Sennheiser HD6xx by how much will depend on how it will sound on my ears, lets see if the burn-in time makes any difference, it did on my Denons.
This review will be updated as it goes.