advice of fellow a'goners regd optimal break in period and is the break in period dependent on playback volume or amount of
gain.
I will re-iterate what I normally say to these threads; well designed audio electronics equipment and cables should NOT drift significantly (i.e. audibly) between the time you plug it in and a few hundred hours. Of course, things age with time, heat, stress etc. but useful product lives are usually measured in numbers of years...
As to reported vast differences after a few hundred hours => either this is perception as the ears/brain are more familiar with the "sound" (it is well known that musical hearing ability can be improved with training/repetition) or it implies a badly designed piece of gear...(linearity, stability and reliability of ouput signal over time is a primary requirement of an audio product)