Try removing the EQ from the setup completely, and see if that changes anything. Are you running anything other than vocals through the PA? If you're trying to mic everything, you might as well give up with just those two speakers. Anything else running through them will pretty much drown out the vocals. If all else fails... Simply turn your instrument amps down and make the drummer use lighter sticks and play a tad softer. 15" speakers aren't the best choice for vocals anyway. They tend to make vocals a bit muddy and woofey. 12" speakers do better for vocals, as they keep them tighter and will let them cut through the mix better.

One more thing... Look to see if the mixer has a pad button on your mic channels. It's possible that they are engaged which will attenuate the signal, which in turn will kill the volume level. If this is the case, simpley dissengage the pad switch, and watch out!