You need more power AD3Three is on the right track. Try and get an amp to power your mains and use the Yamaha to power the monitors. I have a 600 Watt yamaha PA head and I don't think its enough it has two amps that you can split sending 200 Watts power to the monitors and 400 to the mains but it sounds bad that way not enough power to the mains. We have JBLs that can take 500Watts each and peak at 1000Watts so they aren't getting pushed very hard. So try and get a little more power if I need a pa to take to gigs I would have about 1200 watts, sounds like a lot but you can always turn down the master, but a low wattage PA will never get louder, just distort.
Couple other things monitor mix is important try and get good monitors take time to learn how to use them and set them up so you can hear. My brother in Law invested in wireless in ear monitors very pricey but he sounds 10 better when he sings now. But learning how to get the best monitor mix with what you got will go far.
A few mixing tips that might help. If your PA or Mixer has a gain knob on each channel use this as a Master volume for that Channel then Adjust the fader, set the gain as high as it will go then you will have more room on the fader. If you have an EQ set it fat and sing through that adjust the Freq that you feel need to be more present and pull back some of the others.
Reverb! Just a touch to much and you will feedback like crazy, a lot of singers over do the reverb and then complain there not loud enough pull it down and add it to taste.
One last thing think of PA stuff like you would guitar gear, better more efficent speakers are going to sound better. YOu want CLEAN head room so high watts. If on a budget Carvin makes good gear some of the Peavey stuff is good and Yamaha even some of the Behringer mixers are good, becarefull with there speakers I have heard a lot of them sound muddy and tinny. I would spend a few $$$ on monitors and watts the rest is just gravey after that.
Hope that helps.
Hope that helps.