Feedback
Hi. Firstly, you should reconsider your choice of equipment. Those Berringer mic's are pretty poor, as are the apex. I suggest you replace them all with:
Sure SM-58's for vocals
Sure SM-57's for miking instruments/cabs/percussion (if necessary)
Why? These are time tested work horses that do the job well. You can pick them up used for less than $100.00 each. It is useful to have all of your vocal mics be the same brand/model as it makes EQ much simpler.
I would also shit-can the Berringer effects device and mixer. Simply put, they are pretty noisey, unforgiving pieces of equipment. Consider buying a used Mackie 8 or 16 channel mixer. They can be had for a song and you can't do much better than a Mackie.
I'm not so concerned about the brand of your power amp, but its power. 100 watts sounds like a lot until you put it in the context of P.A. My band routinely runs 500 watts to the house and 250 watts to the monitors, and we're an all acoustic band with no drums. The wattage isn't for the sake of volume. It's for headroom, which brings us to the subject of feedback.
Your first step is setting up the room correctly. Make sure that all microphones are on a plane that is behind the main speakers. If the mics in line with or in front of the speakers, you will have no chance of controlling feedback. This is an absolute. The mics must be well behind the main speakers.
Before each gig, you need to "ring out" the room. With all of the mics turned on, your mixer's channel an main faders all the way down, set your mixer's EQ or outboard EQ (great, if you have one) so that each fader or knob (for each band) is fully ON (faders fully UP, knobs fully to the right).
Now, set each of the channel faders to unity (0 db). Slowly turn up the main faders until you start to hear a ringing sound through the main speakers. Starting with the fader or knob on your EQ that represents the highest frequency band, turn the fader down or turn the knob to the left, slowly. At the point where the ringing sound changes for the better, stop adjusting that EQ fader and do the same for each of the others.
The goal here is to find the EQ curve that eliminates the natural ringing in the room when the PA is turned up to nearly full power. At each frequency point on your EQ, when you find the point where the audible ringing stops, take it down one notch farther, then leave it. When you can turn your mains up about half way past unity and not get any feedback, that is how you will leave your EQ settings for the night.
The Next step is to bring up the musicians and do a sound check. If any feedback occurs, isolate that musician, then try to ring-out just his mic or instrument input by finding the frequency range where the feedback is occuring, then train back the EQ fader for that range. There will be some trial and error here.
Once your sound check comes up clean, don't touch the eq again for the rest of the night. During the performance, your sound guy shoudl rely mostly on his ears to set the right levels and may need to make slight adjustments.
If you ring out the room correctly, you will get the most headroom possible without getting feedback.
The importance of having goodly amounts of mains power is that the more power you have, the more headroom you have before feedback occurs.
That's the theory, anyway.
