I agree with Marakanetz' observation that gullible 'philes often won't realize that apparent improvements in sound are really in their own heads, but I think it's both: my listening gets used to the sound of the component and the sound actually changes. I think break in exists, as the heat and current affect brand new circuit elements. This is distinct and separate from warm up, which also exists. It isn't a great step from believing in break in to accepting that it can take longer for some circuits and circuit elements.
I bought a Magnum Dynalab MD 208 last year, a piece that has a reputation for a long break in period. Magnum Dynalab pointed out in a post-review letter that its heavier duty circuit elements required a longer break in. I did notice the sound change. I tried to be as objective as possible when listening, half doubting that I would actually hear any change. I left it on all the time, playing softly when no one was present and turning the volume to zero when the room was in use. I did critical listening only on weekends. After the third week (500 hours), I could hear real differences (improvements) versus on day one. I don't think I imagined it.
I bought a Magnum Dynalab MD 208 last year, a piece that has a reputation for a long break in period. Magnum Dynalab pointed out in a post-review letter that its heavier duty circuit elements required a longer break in. I did notice the sound change. I tried to be as objective as possible when listening, half doubting that I would actually hear any change. I left it on all the time, playing softly when no one was present and turning the volume to zero when the room was in use. I did critical listening only on weekends. After the third week (500 hours), I could hear real differences (improvements) versus on day one. I don't think I imagined it.