I am also just passing by and say that I too have not heard the 630, but own some Senn Momentum 4’s. While @generic is essentially correct, the electronic gimmickry in the 4’s give it a less closed and more forced open into another dimension sound than most closed backs.
My guess is that similar canoodling has been done to the 630, making them possibly less objectionable, particularly if you are a cyborg.