@monsterzero you are right, this is really one of the few headphones that I've heard that come alive incredibly with the right amps. They still sound incredible out of my O2/ODAC though, and I don't feel lacking at all.
Edit: I was reading the graphs improperly, the HD250 has less distortion than I wrote below.
So
@d34dj3d1, in my opinion, few open headphones have sounded as correct as the HD250 (I prefer these over the open back HD650 but those are also very good). They sound more correct than my Audeze LCD2.
It is hard to be honest
and if I am them, HD250s are among the best headphone I've heard ever. Yes, they have sonic imperfections, but their frequency balance is where it matters most. I'm listening to the new Tangerine Dream album and they sound great, as they would with Youth (dub artist / also of Killing Joke), then to psychedelic rock, and any electronic. I feel like I'm in the room.
These headphones are a deep listen. But they absolutely will need an additional amp at minimum an Objective 2 (O2) or similar.
So with all that said, I'll still recommend the HD630VB. Especially, if you can find it used. This is incredibly well-engineered headphone, and technically is better than the HD250. It is stunning what can be pulled from these headphones. So overall clarity of the HD630VB is just ahead. And the bass with 630VB matches and surpasses the HD250. Really, either of them would be great. The HD250 for strict natural tonality, (the HD630VB is very close in this regard though). The 630VB sounds almost 'bell-like' in its clarity, that if you were inside of a large bell and someone struck the outside this is what you'd hear - all the resonances and nuances.
The 630VB you won't even need to get an additional amp/dac. The HD250 will be more comfortable - less clamp though still some, easily obtainable earpads (though seal is important). The 630VB practically gives my a bit of the HD250 on the go, anywhere I want to listen. That is the achievement.
Still, I won't lie - the HD250 are still .. damnit . Better. They are that headphone you can listen to for hours. The HD630VB is just as good in its own way but the 250 have that rare magic. Ultimately, would recommend both.
Quite torn between them, but the 630VB is very good, but just a bit sharper (so that may or may not appeal).
I feel this sites' measurements are accurate.
https://diyaudioheaven.wordpress.com/headphones/measurements/brands-s-se/hd250-linear-ii/
https://diyaudioheaven.wordpress.com/headphones/measurements/brands-s-se/hd630vb/