Up to about 60-65% of max volume, both sounded just fine, but turning the knob above that did produce some clipping or distortion. I would say however, this is at volume levels way above anything I would listen at. Perhaps the PA2V2 doesn't have enough juice above ~75% of max volume to drive the HE-400s cleanly? Not too surprising as planars are (generally speaking) thirsty beasts.
I guess that seems to be the case then. The Fiio E11 seems to have more power than the PA2V2 because I can listen to much louder volumes on the E11 without any distortion or clipping using high gain.