I wonder if the significance of "burn-in" (or perhaps "break-in"), depends on the design of the speaker, especially the material used in the cone surrounds. Some materials may become more flexible with use.
As I have said before, the theory of break-in also suggests the speaker makes a measureable amount of wear. If my speakers have a measureable amount of wear after, say 100hrs of use, how bad are they going to be after 200 hours of use, twice the amount of breakin/wear, the tonal changes should be just as much after 200hrs as they were in 100hrs and imagine the amount of breakin after 500 or 1000hrs of wear. I would hope most speakers built these days are able to withstand the test of time better than that.
I believe if there is any actual speaker break-in it would happen from a few milliseconds to a minute or so, just to unseat a stuck voice coil or something, possibly due to shipping or poor parts; and that would indeed be a noticeable change.
