As the headline implies, after years of using the HD600 headline pad, I searched around and found that the HD650 pad was compatible.
So without any doubts, (thinking it could be worse than the stock ones) I ordered it along the foam pads and to my surprise it's super comfortable!
After less than a week (Thomann shipping is always incredible), I managed to mount them easily (there are YT tutorials for it, that way you make sure to don't screw up anything), and I can't get enough of them!
It's almost like not having headphones on top of your head! Clear and well designed, useful for long productions/mixing/mastering sessions.
Well done Sennheiser, kudos.