phantom power is for powering condenser mics. so if you have a condenser mic plugged into the pre, you would need the phantom power on (on the preamp). the pre would then feed a line level signal to your mbox, no phantom power necessary on the mbox.
mics are pretty finicky devices, especially for voice, and i know absolutely nothing about your genre. you may want to hit the rap/hiphop forum for advice on that one. good luck man.