Me and my drummer friend try to have some rehearsals and create music together. We have a basic equipment, but we can't get to the point where vocals would be loud enough to play with drums, without constant feedback. Now we want to upgrade our setup, but we don't know what would be the best choice.
Here is our cheap setup:

Vocal microphone: Behringer XM1800s
It is connected to the powermixer: LDM SKM-7c.
Here is its site, unfortunately couldn't find it in english http://www.ldm.com.pl/pl/archiwum-pl/kolumny-aktywne/skm-7c-10-detail
To make it louder we tried to plug the powermixer to the 150W Line 6 Spider guitar amp, but it creates even more noise. What we already did is arranging the setup so the microphone is on the opposite site of the room to drums and the amps are directed away from it. Here is the scheme of the room:


The powermixer is set at about half volume, and produces feedback beyond it. It can be barely heard through the drums and we think that cranking it up to 3/4 should be enough.

Now we want to upgrade the setup a bit and we don't know what should we invest into. The quality of most of the equipment is low, but we can't upgrade everything at once. The question is:
Would a better quality microphone like Shure SM58 eliminate feedback, or should we invest in some noise gate or try to find another mixer, or a preamp for a microphone. It's worth mention that our current mic doesn't seem to eliminate much of the sound coming from its sides. It's about 50% of volume compared to sound coming from its front and I guess the sound coming through the sides might increase the feedback massively.

