After reading this thread I am surprised that there are still those out there that adhere to the idea that break-in does not exist.
Some years back we looked into the phenomena to try to figure out what was going on. As a manufacturer I can tell you that we hear break-in all the time- the question was what was causing it. Here's what we found:
built up, one of our units has specific power supply voltages. After break-in (6 months), the unregulated power supplies will measure slightly higher. So something in the power supplies gets more efficient. The appearance is two-fold: filter caps seem to operate more efficiently, although they are 'formed' at the factory. However after break-in the current inrush at turn-on is lower! The other appearance is the transformers themselves, which have substantial lengths of copper wire in them- which has a measureable break-in: lower resistance.
Resistors do not appear to break-in.
Film caps do! -and their qualities in this regard vary according to dielectric and manufacturer. Oddly, we have seen a lot of comments about the V-Cap Teflon (probably the best coupling cap out there right now) breaking in, but we do not find that there is a break-in with it of any significance, which seems to be common with other Teflons we've used. What *does* seem to happen is that the installation of the part can disturb other elements and the part itself is so transparent that it reveals that easily.
Wire does seem to break-in. Copper in particular- silver does too but is much faster break-in, although they both ultimately arrive at the same place if the silver is pure and the copper is pure, and both are installed with correct attention to details of use specific to each.
In all cases, the results are measurable and audible, in this way confirming their reality and not some sort of myth.