can the pre amp out jack be used solo as a "line out"? or does it have to be used along with the power amp in jack like a loop?

right now i'm using the in-out's as an effects loop, but i'm wondering if the amp can be plugged into a mixer. the manual says it can, but i can't seem to get a signal on the board from the pre amp out. not much help in the manual.

_________________