Here's something else I've noticed...when I listen to open-back headphones, I will occasionally hear something that I think came from outside my room or way behind me that will make me take my headphones off and look around to make sure it wasn't. That's what I mean by open soundstage, where it feels like the music is coming from the room you're in and not what you're wearing around your head. That hasn't happened to me a single time with the Expanse yet. I'm always very aware that everything is playing out between the headphones right around my head. Even with the most spacious of albums I'm familiar with, like big binaural albums like Yosi Horikawa or some Chesky recordings.
Not only is the soundstage smaller on the HD6xx for an open back I also should of mentioned that without music playing it dampens ambient noise by like 10-20%. Its that mesh inside the headphone that is doing that, IMO. So that wouldn't be amp dependent. I think that is what people call the "veil" possibly. Its not like they can't have those out of head experiences, but depending on the song other types of headphone can do it more so.