speaker break-in, how loud?

NO, you speakers wil not suffer negitive effects from underdriving them. Only clipping from your amp or overdriving them will damage them.

NO, just play them at the volume level you would generally listen to music at. B&W recommends 15 hours of break-in, which means just playing them for 15 hours. However the Kevlar driver, IMO needs more than 15 hours. They will sound better at 15 from the box, & improve at 50/200/500 hours levels. As a B&W junkie for the last 12 years, I still have a pair that old that still sounds fantastic! I generally do not listen to them to critically evalute them till after 200 hours. Enjoy!

Surely you shouldn't drive your speakers to clipping.
I'd say to stay aware of initial "critical testing" for 50...60 hours.

What you are breaking-in is copper,in the binding posts,internal wire,drivers,crossover parts. This normally takes about 330 hrs. of music signal played at normal listening levels.

Oh c'mon. Copper is hardly the only part breaking in with new speakers. Consider the rubber driver surrounds, drivers themselves, and driver - voice coil assembly for starters. There is more to audio than metallurgy.

Flex is right.
The break-in is mostly on the mechanical domain rather than contact or electric.
It's sort-of proccess of gettin' used to stress in the voice coil and diffusor mostly.

What about In the case of a hybrid electrostatic and dynamic combo? I won a pair of Martin Logan EM-ESL hybrids with an curved elctrostatic panel and an 8" Woofer . Should I play them at medium levels continiously for 200 hours, or at various volume levels from ppp to fff, with 1/2 hour breaks, over the same amount of hours?

Any idea how long a hybrid Dynamic/Electrostatic speaker would take to properly break in, specifically Martin Logan EM-ESL's?

First two days 50-70MPH. Later faster

Have any of you taken apart a speaker driver to see what's in there? Surrounding and supporting the voice coil, you'll see the "spider". Think of it as the other "driver surround".

Typically, this is made from epoxy-saturated cloth, and pressed into shape. Over time, the spider develops micro-fractures that makes the spider more pliable. One of the reasons you're not recommended to "break in" a speaker with a single tone (sine wave) is you want micro-fractures to occur throughout the spider to support the entire bandwidth of tones the driver will be asked to reproduce.

The spider is far more (but not always more rigid) rigid than the typical foam / rubber surround simply because of the mass of the item - the voice coil - it's supporting.

Yes, yes...and you're "breaking in" the copper in the binding posts as well. That is...if you believe in "speaker break-in".