Thanks Gary, the kind of feed back was a low rumble type sound- not the typical high pitched ringy sound.

We use a Shure SM 58 and Peavy i12 cardioid.I use the Peavy and run it through my tyros and my wife has her Shure going through a TC Helicon Voice Live 2 and then into a yamaha mixer.

I'll play around with the EQ and gain. We've only had this problem at one place that we've played. Maybe it's the room. I didn't have time to do a good sound check.