09-12-11: Almarg
I would just add to what has been said the thought that if there are in fact mechanisms by which warmup beyond the point of thermal stabilization can produce audibly significant changes, I see no reason to expect those changes to necessarily be for the better. And I would certainly expect both the magnitude and the character of the changes to be dependent on the specific design.
a lot (in fact, i suspect the great majority) of high end audio is sold to people who know little about electronics. so in some regards, the most important feature of high end audio products is marketing: because it is through marketing that you make the suggestions that influence consumer perceptions about audio products. you can convince people to believe just about anything with enough suggestion (e.g., a surprisingly large percentage of people still believe that saddam heussein was involved in 9/11). to the extent that people believe that there are sonic benefits from keeping equipment on 24/7, then that's just what they believe. however, other than for the possibility of bad system design, i see no scientifically valid evidence to support such beliefs.
first, while it is true that the electrical characteristics of semiconductor devices does exhibit some thermal dependence, it is also true that semiconductor devices are characterized over a temperature range that goes from well below zero to well above boiling point. so i question whether there really is that significant a difference in the sound of audio electronics over the operating ranges of these devices.
second, even accepting that there might be some audible difference in audio electronics during the "warm up" period (and, btw, the testing methodologies to evaluate this are *highly* unreliable), what really counts is the junction temperatures of the devices, not the ambient temperature that you feel when you touch the heat sink. the junction temperature would rise much more quickly than the ambient temperature, so i would expect the devices to reach their "warm up" within 15 or 20 minutes at most - not over a period of hours.
i would find a more persuasive argument that the sound character of the speaker changed (for largely mechanical reasons) during the course of operation.
as to the concept of "break in": the term, as audiophiles use it, is really a psychological process, not an electrical process. it reflects the time that it takes for you to get used to the sound of a new component. when you install a component in an audio system, it will likely have it's own sonic signature. when you first hear it, you will likely be very sensitive to the differences in sound relative to that to which you may have become accustomed. however, over time you will get used to the sound and will become less sensitive to it. as an experiment, change the loading and/or gain on your phono stage. depending on how much of a change you make you should observe a noticeable difference in sound. if, for example, it sounds too bright at first, over the course of time, and as you get used to the sound, you may perceive that the sound is less bright.
you've just done audiophile "break in" on an existing piece of audio equipment...