The mixer does make a difference when it comes to quality because obviously the main preamp and channel strip is still from the mixer and the quality makes a huge difference. Plug into a midas and leave the equipment flat will sound better than a cheaper mixer hacked away. You will have to get yourself a graphic eq to get rid of the feedback without killing everything but again depends on the quality of the graphic. You can then just eliminate the problem frequencies e g 1k, 8k etc. You don't want to take out too much on the channel strip or you will just kill the whole sound unless you have a mixer that has sweepable eq. With a good eq the mic position won't matter. Hence extremely loud stage monitors with a mic on top of it with no feedback it's all about taking away the problematic frequencies.