@erik_squires : Much different situations effecting the changes in these devices: Speakers, being electromechanical devices, will change continuously (usually for the worse). I remember an old Western Electric film (yes film, not a video) showing slow motion photography of an operating speaker. It clearly showed how the driver's cone rippled and distorted throughout each excursion (even at low amplitudes). It's no wonder that over time the light cone materials become less ridged and far less efficient at moving air.
Capacitors have been found to change in several ways: actual capacitance value can change due to physical changes in the shape of the dielectric material (the capacitor may swell) or because of changes in the properties of the dielectric, generally caused by decomposition due to heat-induced chemical changes. The latter problem is especially inherent to the paper dielectric material found in those boutique audio capacitors we are so fond of. Another change capacitors are prone to is leakage of DC current. This problem, due to such things as chemical changes caused by the breakdown of the adhesives in the dielectric, will (over time) cause the capacitor to conduct enough DC current to cause changes elsewhere in the circuit. This is especially troublesome when coils, like output transformers, crossover inductors, and speaker voice coils, are subjected to excessive DC current.
But, during all the degradation and chaos our sound reproduction systems are vulnerable to, there is one constant we can depend upon...the resistivity (the measure of current flow through a conductor) of the wire connecting the component parts of our machines, will remain remarkably constant. This is probably why most of the reputable manufacturers will test and "burn-in" components such as tubes, transistors, and transformers. Some will even burn-in passive components, like chokes, power resistors, and capacitors used in the signal path... but NONE of them burn-in the interconnect wire.
