Juggernaut,
The white noise at full volume setting over headphones is normal "thermal" transistor noise and you'll hear some of that over headphones from any solid state preamp or amp aat full gain or volume. However, hum should not be present at all. Did you disconnect the separate ground wire to your power amp when you did the headphone hum test?
If so, and the hum is still present in the headphones, then yes, I think the preamp's power supply board is defective or there is a cold solder joint on a board. If it's under warranty, let the service guy find the problem and fix it. If the hum in the headphones ceases when you disconnect the ground wire to your power amplifier, then there is nothing wrong with your preamp.
Solid state preamps using the headphone jack should have NO audible noise or hiss at normal volume control settings.
