It may be that because your performers are a bit reluctant you're having to turn things right up to get any kind of sound. They're relying on hearing the main speakers. If they can hear what's coming out the speakers them so can the mics. Bad combination.
One possible (unconventional) solution that might be appropriate if the room acoustic is reasonable as you say:
Buy or put together some long XLR leads and mains extensions that allow you to put the speakers at the back of the audience (sensible cable routing please). This might sound odd but it's a bit like what people do in rehearsal rooms.
Performers hear a realistic balance and are more inclined to 'speak up' when they realise they can't be heard. Audience will soon cease to notice the speaker positioning. Speakers are far enough away from mics not to feed back so easily. Speakers are also facing the 'null' point of the mic where it is less likely to pick up the sound.
I've used this setup several times for a variety of reasons. If it suits your room and your people the fact that the audience and the stage are 'sharing' the same sound can make for a nice ambience.
Ohm's Law states, "Your PA isn't as powerful as you think it is".