I don't know if they normally require a break in period but I will say my 760FLs sounded better as time went on...after a year I really liked the sound and loved the feel.
However, D broke and I kind of wanted to try something heavier. I just put some 760FMs on and they definitely have more tension, I'm kind of hoping they will loosen up a bit in time (like the 760FLs did). The tone of the 760FM is dead on for what I was looking for. The 760FLs weren't quite there.
...which means the 760FSs might be really good since they are in between
