Keep main speakers turned away from mics. If you are running a monitor system you must eq it. EQ-ing you monitors is perhaps more important since the monitor cabinets are so close to the mics.
Notice a sort of echo-ey resonant sound on the low end? This is a low end feed back. Very annoying. This is around 150 htrz or so. This is called a standing wave or eigentone (sorry for the spelling). This is the most inefficient area of speakers except for RTR speakers, but then who has the mony for RTR"s!? Drop your EQ on the monitor side around this range (150 give or take) about -4 to -6 decidels.
So-called cardioid mics are not as good as claimed, unless you get one that's about $300-$500 range. This info is straight from the man who invented the cardioid mic. I used an omni-directional mic in my last band. I could turn that puppy up at least 1/3 louder than the other guys' mics before feeding back.
So- EQ for the main side of your PA- color the sound to your taste and the room qualities. A second EQ- monitor side- to control feedback. Don't worry too much about tone quality. All you need is to hear yourself clearly. Usually you will notice there is no reverb on teh monitor side. Reverb is basically- controlled feedback.
I did sound for a band once this way. All we had at the time was a funky 200 watt PA, but with my setup it was louder and clearer than the previoius band which had Crown amps!!!! Why? EQ control and my ears. Ears are your best pieces of equipment! Wouldn't you all agree?!
Sorry for any typos. I can play bass well, but my typing sucks!