I posted this in another thread, but this is what Eric Alexander has to say about break-in:
The full-range mystery: The break-in process (with all of its negative attributes) simply happens to be most intrinsically connected to, and is most audibly discerned through a full-range transducer. This is because a full range simply has the ability to augment the very subtle higher frequency centered distortions, squeaks, and squawks that a crisp new speaker is capable of producing. Once again, if it is a full-range speaker and if it sounds bad out of the box, don’t wait for a miracle to happen; send it straight back to whomever you got it from because the speaker has serious problems. If it sounds quite good but has a very subtle graininess, or a few mild distortions, or the sound is crisp but still pleasant, then you’ve got a keeper! Sit back and be patient, and you will be pleasantly rewarded within 15-20 hours of moderate playback levels – this amount of break-in time will get you to about 80%-complete break-in. Beyond that, a functionally full and nearly complete break-in period of 200+ hours is quite reasonable and is to be expected.