In reply to:

As far as I can see the break-in phenomenon / myth seems to be limited to metal drivers.

This makes absolutely no sense to me at all. The only difference between a metal-coned driver and a non-metal-coned driver is -- you guessed it -- the cone material. If ANY component of a speaker driver experiences break-in, I would think that the cone is the least likely suspect. And it it's a METAL cone to boot, it's more rigid and less susceptible to change over time.