If your studio is acoustically treated and still not good enough, you can try two very basic things:
1) remember to put the microphone against the monitors (so that the singer looks into the monitors, while the mic is facing away from the monitors). I'd try putting the red monitors next to / below the violet ones. As you have it now, you have four sources of sound, coming to the mic from many directions, generating superfluous sources of feedback you need to fight.
2) position all speakers so they would play into the ears of the musicians. If, for example, the guitar amp is small and is lying on the ground, and the guitar player is standing right next to it, then the guitar player will have to turn the volume way up so he would hear himself, because his amp's speaker is generally 'playing forwards' while the player is 'standing sideways'. If that is the case, then probably the guitarist hears less of his guitar than the other musicians. What to do: put the amps (and the guitarists) on the chairs. The closer the guitarist's ear to his speaker, the less he will need to turn the volume to the right. But be reasonable, hearing damage is a real danger. The same is valid for bass player of course. As for the singer, make sure the monitors are in the height of his ears.
Good positioning of amps and monitors is as crucial as good room dampening. It surely is pretty annoying to reposition the monitors, stands, all the cables... but even a small move might sometimes solve the problem, while buying more electronic stuff only softens it. Try new ideas every rehearsal, until you find it.
To find the right voice volume, start with just a core: one mic, one 300W monitor, no gEQ... does he hear himself? No? Then add the second 300W. 2x300W for just a voice should be more than enough for rather any given room. I think that the violet monitors might be completely superfluous (turn them off, if you hear no difference in volume, forget about them). If you are successful with fighting the feedback and the voice is still too silent, you might consider using some voice preamp to better control the mic gain.
I came to understand that with high volumes, the ear goes numb after a while. So you increase your amp volume... etc. Nowadays, when I don't hear myself, I turn the volume down. It helps surprisingly often. In my experience, this method is hard to explain to guitarists somehow. You could try to tell them: 'If you want to kill with the volume, position your amp so you would kill yourself and not the others.' It might even help them to develop their musicality.