Hi Joost. What I've been playing around with the last few days is routing the vocals out of output 3 into my Scott power amp to get a separate mix of just the vocal output. I really need to pick up a compressor to squish my vocals into behaving themselves. My wife's voice projects beautifully while mine keeps getting stuck somewhere down the mic cable.
A separate amp dedicated to the vocals seems to work well. That way, I can set the arranger volume through the internal speakers and have plenty of volume for vocals without them feeding back through the internal speakers. I'm still not completely satisfied with this arrangement. But it’s getting close.