Critical changes occur with tube age. Almost any tube will need to burn off metal oxides. The difference in sound is usually a less strident sound. When the tube goes through this initial process they tend to provide a nice plateau if its the right tube for you for a long time and then go through a period of real mellow musical bloom. But nothing lasts forever and the tube starts to announce its death with crakles and pops or just blows all at once.
