One word for these headphones: WONDERFUL.
I owned a pair of HD650 back in 2008, in a period where I tried a lot of audiophile grade headphones. The HD650 were by far, my favorite.
In my search for audio nirvana (not really, I was just trying a lot of stuff), even loving my HD650 so much as I did (to the point that I was anxious to get home, to relax and listen to music through them), I sold them in order to keep buying and trying more stuff. I missed them since day one. I should not have sold them.
At that moment the budget did not allow for a solid headphone amp to pair with the HD650, but I had a portable headphone amp that I think is not manufactured anymore, the Headphonia Lyrix DAC-AMP. It was great. I always wanted to try these incredible headphones with something up to the task of properly driving them, making them shine, because most experienced users told me that I was missing out. A couple years ago I decided to buy the HD600 and I paired it with th Schiit Stack. It was terrific, truly great, andthen I upgraded to the Valhalla 2 and it was even better. Even like that, I felt by memory that I enjoyed the HD650 more, and was determined to get one. I had a very clear idea of what I would consider better sound in the HD600: the voices sometimes were overwhelming and the soundstage was very tight. ThenFound out about massdrop HD6XX and decided it was THE time to get it.
WOW. I have a gaming setup with lots of cables and in the center there is the HD600 with the modmic 4.0. Even as great as it is, to listen to music and relax I always grab the HD6XX. It is the HD600 with all the detail and even more, but voices are more laid back, and the sound stage is amazing. Amazing doesnt mean artificially big. You close your eyes and usually can feel you are in the middle of a great bad playing live in your living room.
200 for these HD6XX, 389 for the Valhalla and 150 for the modi 2 uber and you in for a treat and one that would require a lot more money to improve just a bit. An answer for a question that keeps being asked: The extra money for the Valhalla 2, is worth it? It is three times the magni 3 and it is not three times the sound, but it is worth it IMHO.
One important thing about another question that keeps being asked: any of the HD600s sound very nice out of a headphone jack. Sometimes I sit down and plug the hd600 directly to the mixamp pro (which has the output of many portable devices), and the sound is very good. To get greatness from the headphone you need an amp, and believe me, it has almost nothing to do with sound levels. An amp provides a lot more than that.
Grab one and be delighted.