Whatever your setup, if the mic can “hear” the loudspeakers, you’re likely to be fighting against getting feedback.
Sometimes EQ-ing the signal to the loudspeakers (to cut around the frequency of the feedback) can help, though then you’re not hearing what you really want to. Moving the mic a little might work, or might just change the pitch of the feedback. The best thing is to try to prevent the loudspeakers’s sound being picked up by the mic - eg using a very directional mic and with a barrier between the speakers and mic, and perhaps another barrier to block sound reflected from walls – but that, too, might not be successful.
A completely different approach that I’m aware of but never actually tried was a setting on the Eventide H910 Harmoniser, which was supposed to be able to reduce feedback significantly by introducing some kind of variable pitch shift.
Here’s a relevant reference:
When I was researching the Eventide H910 Harmonizer, I found it curious that the box had controls for both feedback and something called “anti-feedback.” The service manual explains the anti-feedback control as follows:
Increasing clockwise rotation of the ANTI-FEEDBACK control progressively adds a small up and down frequency shift to the output signal, which serves to decrease the effect of room resonance peaks on the signal which ultimately re-arrives at the microphone.
In modern terms, I would call this a chorus effect, with a triangle wave modulator. Pretty simple. However, it is interesting to see how such a simple process can have a significant effect in a PA system – by turning on the Anti-Feedback control, you can increase the gain of a microphone being fed into the H910.
The idea of using a time-varying system, such as pitch shifting, delay modulation, or frequency shifting, to increase the maximum gain of a system before oscillation occurs, dates back many decades. {…]
… Feedback, anti-feedback, and complexity in time-varying systems – The Halls of Valhalla
Perhaps there’s a modern equivalent? - perhaps something that could be done in software, eg as an insert on the Cubase output feeding the monitors?
I don’t think the author of that quote was quite right in describing the process as “a chorus effect, with a triangle wave modulator” - IIRC, that’s not how it sounded when you engaged that switch on the H910. But perhaps, in the absence of a method based on pitch-shifting in software, you could see whether a chorus effect or some kind of rapidly varying delay effect might kill the feedback without changing the quality of the sound too much? I haven’t set out to try anything like that - just suggesting something that might be worth trying if you do continue to get feedback after getting new equipment.