Ok, a brief update about the ANC on the Momentum: I have just spoken with their German Support, and, no, the ANC is not integrated in the BT chip. It is possible to make it deactivable. They just decided not to.
But he told me that it should/could be possible to introduce this possibility with a firmware update, so that a combination of button would activate or deactivate the ANC.
He also confirmed my theory, that the distortion on bass heavy tracks in case of bad sealing when chewing or moving the head depends on a conflict of the ANC trying to treat the music as if it was external noise. That's also why he agreed that making the ANC deactivable would be a good idea.
About the problem with the 3way multifuntion button, they suggested me to make a reset (which is what on page 29 of the manual is called "clear the pairing settings", and which is not well explained, because to enter in pairing mode you must NOT turn the headphones on first).
It did not work. So, yeah, it is a defect unit. Can happen.
I am comparing them with the On Ear version.
The On Ear has got less soundstage, a more engaging/aggressive sound, and a more punchy than boomy bass.
I personally like both very much. I suppose that they should be not only chosen according to the dimensions but also to which music you listen to and which signature you like.
But I will write more on that in my comparison thread in the next days.