I have to throw my $0.02 on Carl Sagan's excellent essay that Ken linked to above. As usual, the essay is wonderfully articulate and accurately conveys to general audience the concept of healthy skepticism which is so important in all scientific thought processes. However, as much as he is methodologically correct, he is still proclaiming his own "faith" when he categorically states: "Claims that cannot be tested, assertions immune to disproof are veridically worthless, whatever value they may have in inspiring us or in exciting our sense of wonder" and "Once again, the only sensible approach is tentatively to reject the dragon hypothesis, to be open to future physical data, and to wonder what the cause might be that so many apparently sane and sober people share the same strange delusion."
As I previously stated elsewhere in this forum, it is usually impossible to scientifically prove that Phenomenon X does NOT exist. All science can do is to demonstrate that you do NOT HAVE TO assume that Phenomenon X exists. This is the true essence of null hypothesis. Whether you still believe/think/feel that Phenomenon X exists when faced with "negative" results belongs to the domain of faith, not science.
Sagan's problem was that he had tried to assert his own worldview as a universal truth, based on the same methodological approach described in this essay -- obviously, however, the fundamental worldviews do not belong to the territory of scientific methodology. In addition, in my (rather radical) analysis, Sagan was so much entrenched in the don't-believe-it-until-proven slogan that it might have acted to deprive himself of true "imagination" as a scientist. Indeed, he was truly a great educator and visionary, but as a research scientist, he did not make a single breakthrough discovery.
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Enough about Sagan; back to audio... My own view is that there is NO SINGLE "PUBLIC" DATA available, either objective (measurements) or subjective (listening tests), that convincingly demonstrate that there is a phenomenon of long-term speaker break-in (over 10's and 100's of hours), or that there are readily audible "sonic signatures" or sound differences among today's good-quality solid-state analog amps ("mid-fi" to "exotic high-end") or among correctly chosen, good-quality cables. All available is, in my definition, PERSONAL ANECDOTES rather than public data -- including all the "expert" subjective reviews published in reputable audiophile magazines.
I repeat that this does NOT prove that there is no audible difference among these gears. It belongs to your own faith how to take the available information. Science tells you no more here.
My own personal faith? -- Basically, I am neutral. While I've never personally experienced or been convinced that these "controversial" differences exist, I would like to be very careful in proclaiming that there SHOULD BE no differences. It also depends on the complexity of the system in question. At any rate, the cerebral geek in me always tends to demand a reasonable, logical explanation when it comes to one of these controversial topics. Incidentally, I don't have a 100% faith in my own ears, either -- while I do wish to believe what my ears hear, I always try to take it critically. If there is even a slightest doubt, I postpone to draw a conclusion. I am too aware of the well-established psycho-acoustic effects to blindly believe in my own ears.
My own personal anecdote? -- To be honest, I always wanted some assurance that my "mid-fi" electronics are not audibly degrading the sound quality of my system. Through this past 10 years or so, I did bring in my home several "higher-end" amps from dealers and friends, and compared the sound quality (instantaneous A/B whenever possible, but not always). The brands I've listened to in-home include Krell, Sunfire, Theta, and Simaudio -- some of these were VERY expensive. Again, these were never a blind testing; I knew what I was listening. In no case, however, I could convince myself that there was substantive difference in sound quality. Otherwise, I would have already bought some of these. I stopped doing this in the past year or two, simply because I've been "discouraged" enough.
In contrast, I am so fascinated with loudspeakers, because different brands sound so obviously different, even among very expensive ones. And I see every physical reason why speakers sound so different from each other.
