Time, maybe 15-20 years down the road your speakers may lose some resiliance. I highly doubt there is any break in period from when the speakers arrive at your door and you first turn them on before you can start enjoying them to their full benefit.

In fact, my 80's Pioneers sound the same as the day I bought them, but of course I'm sure my brain is broken in. \:\)

M80s VP180 4xM22ow 4xM3ic EP600 2xEP350
AnthemAVM60 Outlaw7700 EmoA500 Epson5040UB FluanceRT85