The problem with blending the on-boards in with the PA is that you can sometimes get some weird phase things going, or emphasize certain mid-range frequencies which tends to lead you to adjust your mix for what YOU are hearing, not the audience.
But yes, it can work in a pinch if you cannot set up the PA behind enough, you just have to be super aware that you aren’t hearing what they are. And, like I said earlier, they are very unlinear... if you turn them up, you aren’t turning everything up equally, just one more thing to have to compensate for.
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An arranger is just a tool. What matters is what you build with it..!