When I worked for Cello (Mark Levinson) back in the early 1990's - we wouldn't even play a pair of speakers that hadn't been broken in.
I can't give an engineering answer but I can tell you that the drivers sound better when they have a few hundred hours on them. Perhaps they get into the specs they were deigned to produce.
I also NEVER evaluate electronics cold. When doing the Krell Evolution review http://avrev.com/equip/krell_evolution_system/
I ran the suckers for quite a while before really listening. I wil admit that breaking in electronics is a bit more black magic than speakers but I can hear the difference and that is all I care about.