the PL tends to be slightly forward in the highs but because its tubes that adds no irritants, just a different tonal balance. It is extremely revealing, so you may be taking a layer off your source and seeing a problem that was covered up before.
My experience with break in was basically that the amp got more dynamic, better sound stage as it broke in but tonal balance didn't change.
My experience with break in was basically that the amp got more dynamic, better sound stage as it broke in but tonal balance didn't change.