use the preamp out jacks. You can use either volume control. If your preamp is remote control use the vol on the headphone jack to set the gain permanantly, and use the preamp from then on. Think of the headphone amp as just another amp, and all will become clear.
PS- if you set pre-amp low, the loudest you'll get is low.
PS- if you set pre-amp low, the loudest you'll get is low.