Personally I have no reason to believe that Mackie has some how discovered the secret to the mythical "feedback free" floor wedge.
Any device that recreates sound also has the potential to generate feedback. Feedback is a huge topic and there are already numerous articles all over the net discussing causes and remedies. The easiest explanation and where I am going to leave it is that "feedback" is caused when a signal is fed back to it's point of origin, be it from a Mackie monitor or a Peavey monitor or any other make. Of course all monitors have their own characteristics with certain frequency peaks, but that's why you have an EQ isn't it?
Room acoustics and speaker placement play a just as big of role as anything else in the phenomenon known as feedback. Some useful tools to fight feedback are quality vocal Mic's with good "off axis rejection" , thirty-one band EQs, Speaker placement, Monitor placement, Mic placement and a good understanding of how feedback occurs and what frequencies are often the culprit.
Since the beginning of sound reinforcement there has always been a battle between compromising sound quality to eliminate feedback. In my opinion some monitors tend to perform better than others but in the 300 to 500 dollar range they are all pretty much equal. I think with a little experimentation you will be able to make your Peavey monitor behave itself.
(depending on which model you have).