Filter capacitors in amps and preamps have something called 'forming up' where they adjust to the operating voltages to which they are subjected, being electro-chemical devices. This takes some time and until that occurs the power supply will not be acting correctly. Not sure how long it takes, 50-250 hours? but its not that hard to measure that they do change- the power supply voltage itself goes slightly higher and there is less noise on the supply rails.
So that explains part of break-in. But coupling caps appear to have a break-in property as well and to me this isn't as well explained, but plenty of people report changes in coupling caps as they apparently break in.
We hear break-in artifacts in our own gear and for that reason (and to prove reliability) we play the gear in our shop before shipping. The first ten hours or so see the most dramatic changes and this period of break-in isn't experienced by most people as most manufacturers don't ship as soon as the bench tests are passed. We have noticed however that somewhere in the last 20 years that phenomena is vastly reduced over what we used to experience so we suspect that some part we are using is a bit better at its job now than 20 years ago. Generally speaking though its unimportant as we put at least three days of operation on something before shipment. Often we'll run it longer than that- up to a week. But customers consistently report break-in improvements and have been doing so as long as we've been in business, and having experienced it myself many times I just accept that its a thing. I think power supply filtration is most of it.
So that explains part of break-in. But coupling caps appear to have a break-in property as well and to me this isn't as well explained, but plenty of people report changes in coupling caps as they apparently break in.
We hear break-in artifacts in our own gear and for that reason (and to prove reliability) we play the gear in our shop before shipping. The first ten hours or so see the most dramatic changes and this period of break-in isn't experienced by most people as most manufacturers don't ship as soon as the bench tests are passed. We have noticed however that somewhere in the last 20 years that phenomena is vastly reduced over what we used to experience so we suspect that some part we are using is a bit better at its job now than 20 years ago. Generally speaking though its unimportant as we put at least three days of operation on something before shipment. Often we'll run it longer than that- up to a week. But customers consistently report break-in improvements and have been doing so as long as we've been in business, and having experienced it myself many times I just accept that its a thing. I think power supply filtration is most of it.