Audio Systems: Peavy Powered Mixer, xlr connectors, shure microphone

Question
hey man i bought a Peavy XR 8600 about 3 months ago and its got nice volume if you plug a guitar or media into it but when I plugged my SM48 Shure Microphone into it I couldnt get much volume before it screamed with feedback now i know Peavy just came out with this technology so i has to work right? and with 600 watts that should be more than enough volume for vocals to do a Zeppelin gig.  I dont know if this helps but I run this Mixer through Two Kustom KPC15 15" Speaker cabinets w/150 watt handling a piece, my previous 300 watt Peavy XR 400 Manufactured in 1979 was really loud and didnt scream.  Can you give me some tips on this new mixer before i trade it for a new old mixer?  Thanks Man

Answer
Presuming you are using the XLR connectors, the mic and the input channel you used are ok, I would try a different channel, first.
If the unit was used (and it probably is or you would be at the peavey dealer letting them handle this), you may have a bad channel or master output.
Of course be sure the channels are flat before you begin initially trying to set up the input gain for that channel.