09-18-14: MapmanNot to mention the possibility that something else altogether is changing, in the system or its environment. E.g., ongoing aging or breakin of components, seasonal changes in temperature or humidity (see this post, for example), changes in AC line voltage or noise conditions, etc.
... is it possible that both the product and your perceptions are both changing?
As I've said in a number of past threads, in audio it is very easy to attribute a perceived change to the wrong variable.
IMO. Regards,
-- Al