Royce, way back when I was performing with a PSR-500 and PSR-5700 on a stacked keyboard stand, I was using a 600-watt, stereo, powered mixer that had all the effects. Back then, the keyboards didn't have EQ settings available so that's what we were stuck with.
Well, it was a good news, bad news situation, at least for me. If during a live performance I had to adjust the EQ settings, I had to look away from the keyboard and audience to adjust the powered mixer effects settings, which were perched on a stand about 3 feet away and often in a darkened setting - WHOOPS! Sometimes you inadvertently turned the wrong knob and damned near blew the heads off those sitting close to the speakers.
Today's arranger keyboards allow you to do all this on the fly while not interrupting your view of the audience or keyboard. It's so very easy to do and can be done with a minimal amount of button pushing. I've been known to do this while singing and playing a song.
The problem is that room acoustics often change was crowd noise increases, more people enter the room, and dancers enter the dance floor, which is directly in the line of fire for the speakers. Doing this with an external mixer, IMO, is a PITA. It is something that only takes a few seconds on the keyboard.
Good luck,
Gary