As for solid state equipment burn-in, I can believe that there is some initial change, then change for the worse 10-15 years later when the caps dry out. Whether or not the change is entirely audible is another matter.
I'll be modifying a mass-market grade SACD player shortly, installing new capacitors (BG NX-HiQ) on its output coupling caps after the DAC and also after the op-amp. New high speed soft recovery diodes (Harris TO-251) installed on the power supply diode bridges are a possibility, although this could be complicated due to the surface mount board. I'll make an effort to record its output on several songs both before and after modification, as well as weeks and months into the modification. Sine wave tests are possible but not appropriate in this case, since the purpose isn't to establish any technical specs but rather to record any sonic change in music. These modifications are common, and the Black Gates in particular are said to improve bass response when installed as coupling capacitors as they "break-in". If such is the case, the change in bass should not be difficult to document.
