My intention for buying these was to have a solid baseline to compare other headphones with, so my expectations were kind of low. I thought the hype around the 6xx's was some kind of red flag, or maybe it was just people who have listened to bad headphones their whole lives. How could so many people sing the praises of these? It's because the hype you see here and on other forums is understated. The 6xx's delivered way beyond my expectations. They're the kind of good thing that makes you want to tell other people about it, if that makes sense. Rather than describe the sound signature (neutral, spacious, detailed, non-fatiguing), I'll talk a bit about the overall experience.
I had purchased some Audeze LCD-2C's 6 months prior, and was (still am) astonished at how good they sounded. I had no problem being the annoying guy in the office with these open back monsters, but the weight and overall bulk of the LCD-2C's after a few hours became unpleasant. The 6xx's are much lighter, to the point where I sometimes forget I have them on. The sound bleed is something like 60% less than the LCD-2C's, which is a nice bonus. I really like the sound signature of the 6xx's for day to day listening while I'm working or doing something else, so I can pop in and out of critical listening mode. The Audeze's are amazing, but in a way that demands my whole attention.
My setup since getting these has been a Schiit Fulla 2 for work, and a UA Apollo Twin at home. On the Fulla, I'd have the volume knob sitting at 2 o'clock, and still want a bit more. Surprisingly, feeding directly out of the headphone jack on my 2016 macbook pro wasn't bad. I was expecting these to sound terrible without an amp or external dac, but it sounded fine at around 90% volume. I'm listening through the THX 789 right now, and they need the highest gain setting. This is my favorite pairing as of writing this.
Apologies for the ramble, but these are great headphones. They completely solve my issues with using my LCD-2C's at work, and I'm looking forward to getting a tube amp one day specifically to see what it does to these headphones.