I recently bought a 600 watt PA system thinking it would be more than enough for what I need (only for mic'ing our vocalist for band practice). When we first got it out of the box it created a feedback loop almost instantly at hardly 1/10th volume settings. We played around with it for a couple hours and we currently have it working suitably but not ideally, the individual channel level is set to 10 and the master is set to about 6.5-7(for both the monitor and main channels, this is practice so were using them both the same way with no audience). With this setup, however, the vocalist comes through well only if she sings closely to the microphone and if she coughs or anything like that the limiters go way into the red. and this is just for the one channel, shouldn't the pa be able to handle one channel maxed out without going of the chart like that? The two things that immediately come to mind as being problematic are that we are using a directional mic, and we also don't have a windscreen for it, but I'm not sure if either of these things are the root of our problem.
I'm guessing you've got the mics and speakers far enough away from each other they wont be feeding back.

And try backing off the channel level and turning the master up.
My Gear
Cort VX-2V
Stagg BC300
Marshall JCM900 4100 DR
Marshall 1965A
Hartke HA3500 Combo
They aren't feeding back, but it's a small room with far from ideal acoustics, so they are pretty close, though they never actually get sent into a cycle, the worst we get is a bit of a jingle sound after a cough or other loud noise. I'm much more worried about getting a more even volume response so I can turn it up more. And I've tried it doesn't seem to make much of a difference either way, unfortunately :/