I seem to get feedback no matter what I do. I have a Allen & Heath zed 12fx, Sm58's single 15's jbl's and monitors. I get feedback in every venue we play. It's not a constant feedback but a ring here and there to the point it's a problem. I check the gains with the PFL then raise the slider for each channel I'm using prior to playing and they are good. main speakers are out front with the monitors for the band. It's not a constant feedback but an anoying ring. Any help would be a help. How do I get nice punch on the vocals without feeding back? Thanks.