Regardless of other opinions here, break-in is a real phenomena. Those that deny this effect have either never been to a trade-show or don't have much experience with new components IMO.
In new electronics, it is caused primarily by the "forming" of dielectrics in capacitors, wiring insulation and the board FR4 glass-epoxy itself. This forming is actually a molecular change that takes place when voltage is applied. Secondarily, it is caused by excess charge that becomes stored in dielectrics due to temp and humidity changes during shipping and handling. Thirdly, because conductors get moved and formed in cables, they need to "settle" again. This has to do with the molecular crystal lattice of the conductors and their interfaces with other conductors, like connectors.
The second and third effects are well-known to show exhibitors. All systems sound best in a 3-day show on Sunday, the last day, because all of the stored charge is bled-off by then and the cables have "rested".
Steve N.
Empirical Audio