I've got a few issues with our new PA systems we use for the vocals.
One issue is that the vocals are sometimes quite distorted if put too loud, even though the volume is only just past half way.
Also, when volumes are turned just past half way, the microphones are easily prone to feedback when singing in to them. We use Shure SM58 microphones so that won't be the issue. Maybe there are some EQ tips and techniques to conquer these problems?
I play in a four piece male rock band with three singers if that helps too!!
Thanks a lot.
it'll probably be the gain settings that are wrong from what you describe. The gain for the vocal channels should be set so that it doesn't peak even when you are singing your loudest, that normally means you should be about-6db for most normal singing. Reducing the frequencies which are feeding back will also help to reduce fb.

What PA have you got? graphic, parametric or 3 band EQ?
The only 6 words that can make you a better guitarist:

Learn theory
Practice better
Practice more