One of the things that "breaking in" of any component will do is to reduce the high end glare, brightness, shrillness, or what ever term you prefer. The highs will mellow, and the bass will tighten...but the most noticeable effect is in the treble region...at least that has been my experience.
Also, tubes approaching the end of their life span will result in a high roll off and lack of dynamic punch, sometimes, but not always, accompanied by an increase in background noise [hiss].
Also, tubes approaching the end of their life span will result in a high roll off and lack of dynamic punch, sometimes, but not always, accompanied by an increase in background noise [hiss].