You know, I think the PF's are the most sensitive to minor volume changes ear or headphones I have ever owned.
To low and the music loses life and energy, to high and it becomes brittle and uninvolving but just right and it is heaven.
I think it must be something to do with the horn loading, I have experienced this with horn speakers, find the sweet spot and magic ensues.
I tend to agree. Now that you mention it, I realize I pretty much keep the volume at nearly the same level each time I listen.