believe that some electrolytic capacitors need to a number of cycle to fully "form" the electrolyte.. That in itself could account for the difference some people including myself have heard in amps and other electronics...
Overly simplistic and completely overlooks things like propensity of amps and preamps to generated RFI from other components leading to IM distortion, output impedance, stability into given loads and a host of other factors. If a capacitor truly needs to be 'formed', it is done so by a gradual application of voltage which can 'form' the oxide layer on the metal (tantalum, aluminum). To do otherwise is to ensure both premature failure and the inability of the capacitor to meet its stated capacity.
Speakers need some burn-in if only for the capacitors in (most) crossovers.
The manufacturing tolerance in the drivers far exceeds changes that occur in the capacitors even if they're 1% tolerance.
I had an interesting experience with a Magnepan speaker, I think it was the 3.5 which would audibly "flap" in the presence of a loud bass transient (some here would jump at this and tell me there is no such thing) You would hear the panel hitting the magnet... So much that I began not playing certain cuts or albums, one of them I remember was the Dave Crusin Sheffield Direct-to-disc album... After a few weeks of playing same system, same amps. etc..
Why attribute that to capacitors? Why not attribute it to the material tightening up a bit as a result of both environmental and electrical exposure? Sort of like a small amount of residual plasticizer leaching out which makes the material conform tighter. Of course there are bass transients. That's not limited to a particular frequency range but more like rise time.
As some others have noted, they do not notice that their system has changed until they replace their tubes. Those who subscribe to themselves having heard break-in are unable to hear inner-groove distortion in vinyl. No one can balance their speakers to within a few tenths of a dB unless they use a meter. And once again, why is break-in invariably a process that leads to something better? Why don't they break-in for the worse?
Regarding Steve's earlier goading, he didn't hear break-in with Bryston primarily because Bryston did not 'prime' Steve's psyche with the idea. Theta did. Steve, and many of us, are susceptible to this in varying degrees and numerous studies have been carried out in psychological, sociological, and even marketing publications. Priming is the placing of a thought, an idea, into a person's mind without their implicit knowledge that affects their subsequent behavior for a period of time. For example, if you create a sense of urgency in an individual, they are less likely to help someone in need regardless of their profession. There are many other examples. Further, Steve ought to consider other factors, assuming he can, regarding his disposition during various times. Nothing much sounds good to me if I've had a lousy day. I need time to unwind. Maybe you do too. How's your performance in the sack Steve if a case isn't going your way, the weather is hot, and traffic was backed up? Further, critical listening itself can and does result in selective auditory focussing that results in different presentations making its way into long term hearing even though the actual presentation hasn't changed one iota. That's nothing more than the brain/ear reducing the information that impacts it.
Wedding crashers? Moi? I'm not even getting laid here!