I did a quick search and didn't find anything that specifically addressed this question: Has anyone ever found that a product actually sounded worse after it was used for whatever break-in period the manufacturer and dealer recommended? I mean, doesn't anyone find it odd that components always sound better? It could cause a person to wonder whether, to some degree at least, some of the "break-in" is happening between the ears.
All you need to do is to a/b compare two identical pieces of equipment, one brand new and one broken in. I have done that with my Sony SACD player and the difference was very obvious.
I can't recall seeing anyone report that a component sounded worse after break-in. I'd say your suspicions are warranted.
I have never had a component sound worse after break-in. But that's all that means. Not sounding worse doesn't always mean tympanic happiness, for this audiophool.....peace, warren
Try this logic - if you are a product designer, and you know your widget changes its characteristics from its new state to its well broken-in state, do you optimize its performance for the new state, which exists only for a short time, or for the broken-in state, which lasts a long time? I think any intelligent designer would do the latter. I think this explains why some products work a little better after being broken-in.
Nighthawk: Good answer. That's assuming, of course, that the break-in of all the individual components inside a piece is highly predictable. Otherwise, the equipment designers would be guessing -- at least to some degree.

Anyway, I know measurements don't tell the whole story, but I wonder if any of this is measurable. Has anyone done any before and after measurements of speakers, amps, preamps, CD players, etc.? Thanks for your responses.
But how do you predict how a component will sound after break-in, if you don't know what causes break-in? How does an engineer design for a phenomenon he doesn't understand? (Answer: He doesn't.)

As for measurements, I think Tom Nousaine once tested speaker drivers before and after "break-in." The differences were so small that they were swamped by the unit-to-unit differences. I would expect the differences to be even smaller for purely electronic devices.
Components that include elastomer materials, speaker surrounds, and phono pickups can be expected to loosen up, and sound different after some use. Extended LF response is easily measured.

When it comes to electronics, wire, equipment racks, I doubt that anyone can measure any change, although they claim to hear something. Between the ears IMHO.
The "Break-in phono-menon" can be easily explained as a function of the ear to adapt to environ-mental sounds. Whereas the eyes, nose, tongue and tactile do not adapt. Since the function of the inner-ear/brain mechanism is to interpret environ-mental sound it has to change its internal physiology to identify source details. Therefore what appears to sound better is actually your ear conforming to provide you with a more accurate "picture" of the vibratory source, making it more understandable. IN OTHER WORDS YOU ARE WHAT YOU HEAR. This explains why the observed "improvement" is unmeasurabe on lab test equipment, and explains why new equipment never sounds worse. The ear is the only "moldable" sense organ, just as new shoes always feel better after they are worn a few weeks "broken in".
I'm not sure I'd go so far as to say that the ear is the only "moldable" sensory organ. You can become accustomed to hot foods, rotten odors, even touching hot or cold items.

The point is that anytime you do a sensory comparison, the biggest variable in the mix is YOU. You can't hold yourself constant.
Bomarc, you're right, you can't hold yourself constant, but you can maintain a reference sample, as Sidssp said.
Sure, and when somebody compares mint and broken-in components in a blind test and identifies them consistently, I'll believe that break-in is a physical phenomenon. Until then, I'd argue that the psychological explanation carries a lot more weight