There is data that definitely shows changes in the characteristics of the drivers after the break-in; actually hearing these differences is another story.
My point of view is that at least the bottom end should improve a bit; all the rubber parts (gaskets and surrounding of the diaphragm where applicable).
How long is needed it's another issue; I would guess that it varies wildly, and that in some cases the speakers are broken-in when you receive them.
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M80 v3, 2 x Emotiva UPA-1 Monoblocks, XPA-5, UMC-1 PrePro
Pro-Ject RPM Genie, Carver C2 Phono Preamp