...but not the power of suggestion, maybe?
I'm not saying equipment (especially amps) don't benefit from burn-in/break-in, but a lot of what people claim regarding AMOUNT of break-in (in degrees of difference and length of time required) is ridiculous. Yes, capacitors need some time to burn in and settle down, but we're talking larger ones, not the tiny little values in most portable stuff. Headphones, absolutely though...but that's a mechanical break-in (diaphragms moving, voice coils settling, etc.)...and don't get me started on cable burn-in...that's a load of nonsense. Maybe on some fundamental level there is a change, but can you HEAR it, definitively? Gimme a break(-in).