12 o’clock is probably the amp inputs wide-open if it says zero on it there. If so, you would need the MC4000 (your front-end) master turned way down to put the powered speaker volumes at that spot. The metering on the front-end is for the benefit of running the front-end properly; it doesn’t magically prevent all additional clipping or limiting in gear later down the chain. Each piece in the chain needs its own input trim levels set appropriately.
I’d recommend you do put your master at unity on the output of the MC4000 front-end and then the powered speakers’ volume lower, as this maximizes signal-to-noise ratio on both the front-end and the transmission line, as well as prevents users of the front-end from cranking up that master and stupidly crushing into the powered speaker amp limiter dangerously.
Having to put a powered speaker’s volume at 9 o’clock or less is not unusual with a hot pro nominal balanced input signal as you’re going to see running the MC4000 master out at unity and nice proper levels elsewhere on it.
You also might want to ensure the tops are being crossed effectively and not receiving the entire frequency range, as low frequencies hog a lot of power and you want those going to the subs… unless you’re running the tops really quiet, in which case there are ways of more gently crossing the subs at an even lower corner frequency and running the tops full-range (with their natural, inherent roll-off) for less gear in the path. That’s audiophile stuff, though, not PA stuff. You need higher output and gear safety for PA stuff.
If everything else is taken care of and you still see the limiter on the powered cabs flickering, back off their volume knobs substantially from that point so that only in the most extreme overdriving on the front-end would you ever see the powered cab’s own limiter flickering. You don’t want the amplifier limiters kicking in when there’s nothing limiting or clipping on the front-end; that way you have some safe headroom on both the front-end and the powered speakers.
Most importantly, though, listen to the speakers. If they sound like they’re stressing something, you might be doing thermal damage to them even without amp peak limiters on them kicking in. A lot of these class-D amps now will even fail before the drivers will, and the risk to drivers is mostly thermal, too. So definitely don’t put the drivers or the amps into too much stress.