You are beginning to see how small speakers in a TOTL arranger are more of a problem that a help...
There is just NO WAY you can set up a registration to sound it's best on those tiny speakers (I bet you car system has more speakers and more wattage than your Korg!), and, at the same time, sound the same on a PA with tons of overhead and bass and midrange to spare.
Make your registrations on a decent set of near-field monitors, or on the PA itself, and you'll KNOW it's balanced for the gig. Then AFTERWARDS, you can set up an EQ and compressor Master preset to compensate on the internal speakers, which can be easily removed for live use with the PA. This is a LOT easier to do than the other way around!
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An arranger is just a tool. What matters is what you build with it..!