Dassy, you don't say what you are recording to.
HD, ADAT, analog tape?
If you are using a cassette based multitrack use one of the line in's and adjust your trim accordingly for the source.
I still have a Casio CTK1000 that I use as a MIDI module hooked to the Kurzweil as the controller.
The Casio is putting out a line-level signal and needs no further pre-amplification to get a good signal level. Your board might be different. I had
a Roland JX-3P that had a switchable output depending on the input. I miss that board. It's practically an antique now.
Anyway, I would think that any Casio is going to put out a line-level signal. Turn your trim control on your mixer to the line position, turn the volume up about half way on your Casio and then refine the signal level on the recorder using the trim and volume control and channel fader to get the best S/N ratio to tape without clipping or distortion.
If your are recording to HD or into a DAW, I don't think you will need a preamp because of the line level output of the board. Someone else will know more about the digital realm than me.
Good luck. Let us know how it turns out. I think you will be surprised at the results.