Dave,
Everything can be measured. We live in the world where physics dominate over voodoo and some established stereotype human beliefs including religion.
90% when you hear people speaking that the speaker sounds bright or dim in terms of higher freequencies out of the box is the matter of personal adaptation rather than break-in when it "starts to sound ok". The midrange problems might have some large loudspeakers that realy need its voice coil to be "massaged" with some low freequencies to make it more flexible along with its massive drivers. To the high quality speakers with 7" woofers the break-in time can be very minimal(from 10 minutes to few hours).
The main role to confuse and convince people takes an advertisement that brings up "professional reviews" that make people believe that after 1000 hours their speakers will start sound better(otherwise there will be too many returns). It's even possible to make people believe to many many different things beyond it especially when it comes to play with money.
