In reply to:

I will reiterate: if perceived break-in is because of sonic differences from previous speakers, then why don't people ever report their speakers getting brighter or losing bass over time?

Well, you don't have a proof that people have NOT reported such changes.

Seriously... People may well have actually perceived that direction of changes. However, because of the overwhelming degree of prevailing "conventional wisdom" on the speaker break-in, they may well simply dismiss their own perception and conclude that the given speaker is simply "mediocre," especially if the person feels that he is an "audiophile" with highly discerning ears. Or, some people may well have actually reported somewhere such changes, but wouldn't refer the change as a result of "speaker break-in" because of the same reason above. Or, even if somebody reported a case of a "degradative break-in," nobody might have paid a serious attention to such reports because of the prevailing view...