When I plug in my HD660S the V30 correctly sets to High Imp mode and sounds amazing with plenty of volume control.
I forced Aux on by first pluging in an extension and for some reason volume sounds lower in Aux mode. I though it went:
normal < high < aux
Believe it's Normal < Aux < High
If the phone detects impedance > 50 ohms, you get high. Can't remember what triggers Aux mode.