If the setup is working ok for you, by all means. What I was referring to with my comment is that the PPM608 produces a maximum of 150 watts into 8 ohm speakers (like your C300z speakers) with 1% THD/clipping. That's not a lot of power. The more instruments you run into it, the less headroom will be available for vocal reproduction.
As far as feedback issues, mic pickup pattern relative to speaker placement is the first thing to consider mitigating feedback. Mics behind the main speakers rather than in front and monitors outside of the rear lobe pickup pattern of cardioid mics are a good start. Good vocal mic technique would be the second consideration. Feedback is far more likely when people don't put their lips on the mic as more gain is required at the mixer to get the vocal mics to usable levels. My biggest feedback headaches happen with loud bands that have vocalists with crappy mic technique where they're too far from the mic or getting off axis with it's pickup pattern. You should never be more than a few inches from the mic except when someone is really "belting it out" where pulling back a bit helps control dynamics. But a properly used compressor can do the same thing with someone constantly on the mic. The last thing for feedback mitigation is EQ used to "notch out" problematic frequencies that want to "take off". This is both for mains and monitors. The PPM EQ is nearly useless for this because it requires precision to get rid of problematic frequencies in a manner that won't suck the lift out of the mix. So unless you're going to purchase external EQs for this purpose and learn how to use them, mic placement and mic technique are going to be your best bet.
