To use an amp or powered monitor, use the "MONITOR Line Out" jack on the front panel if you want only the monitor mix. If you want the mains mix, then you have to use the "MAINS MIXER Out, L/R" on the front panel. These are line-out signals and can go directly into your amp.
If you use a speaker out jack from the back panel, then you will have some problems with the signal; it may be too strong and cause humming or feedback. And it also depends on whether you are running mono or stereo. You might get different results depending on whether you are running mono-monitor or stereo r/l.
Check your channel dials and see where your monitor levels are; that could be your problem. Or, if you are running in stereo, check the pan dials. Otherwise all channels should come over your signal as mixed. Assuming you are running in stereo, you would need to run two cables from the MAINS MIXER OUT L/R if you are using any panning. Otherwise you would just get those channels that are panned in that direction and at that strength - kind of like a mini-monitor type mix.
I run an EV sub out the front, using one of the mains out jacks; I run in mono-monitor mode.
[This message has been edited by cassp (edited 08-23-2008).]