When it comes to break-in I will quote Dug and say, "I'm having two feelings."
I have some balanced Morrow Audio MA4 interconnects. I didn't focus much on the break-in, but here's how I would describe my experience:
The expectation bias was set by their cable owners manual that essentially says that the cables start out sound good, then go through a period where they sound bad, and then they get great.
When I first hooked them up they sounded amazing and I spent quite a bit of time listening. Then there was a couple of days where I wouldn't describe the sound as obviously bad, but I would just listen for a couple of minutes and then walk away. After even more time I started getting sucked into listening again. I can't say that I could tell a difference between. new and broken in with absolute certainty, but I can say that there were a period of time where I didn't find my system engaging when before and after that time I had to pull myself away and this period of time aligned with what Morrow Audio described.
I try to identify a scientific reason for the things that I experience and I've got nothing for cable break-in.