After reading the reviews here and getting a hefty open-box discount, I recently decided to pick up a pair of these. They arrived in like-new condition, and I've been extremely impressed overall.
I have an HD800S that I bought a couple years ago that I run with my RME ADI-2 Pro, and while it was acceptable after the Oratory EQ settings, I could never get the tonality to my liking. The bass was generally too light, and since I'm very sensitive to treble, I always had issues with the treble peaks. Further EQing the peaks to reduce fatigue just made it sound too dull.
On the other hand, the Expanse sounds great right out of the box. Bass is strong and defined, treble is clear but not harsh, and voices all sound natural. I suppose I just like the Harman tuning and the midbass bump. If listening at higher levels, I do like to EQ down around the 10.5 KHz area a few dB to stomp out any sharpness/fatigue, but even flat, it's totally acceptable. I run it unbalanced, and it gets more than loud enough for me. As someone else mentioned, -30 on the RME is a nice regular listening level, but I sometimes push to -20 or so for the types of music I listen to.
I've also run it straight out of a MacBook Pro and iPhone and Apple USB-C headphone adapter, and they both work very well. I have no issues driving them to high enough levels for me.
My only critique of the Expanse is that although they're technically open back, it does feel more like a closed back headphone. This is a big difference compared to the HD800S, which is almost like not having anything on at all. I do like being able to hear my surroundings (e.g. family members in the house) and having my voice sound more natural to myself when chatting in calls (using an AntLion ModMic wireless). Comfort-wise, though, I think I prefer the Expanse over the HD800S since the pads are so soft and with a higher clamping force they don't slip around as much when moving my head.
All in all, an outstanding headphone. While it's very expensive, I don't see myself buying another pair of wired headphones in the next 5-10 years, so maybe this is going to actually end up saving me money!