So now I have my amplifier and my speakers, but I still need to find a good place to take the audio signal from the Microkorg to the Amplifier. I don't think using the headphone output is a good choice, so I had a look in the service manual (which pretty much includes the entire schematic!) and found a spot just before the headphone driver. This spot also allows me to adjust the speaker volume through the volume potentiometer.

I opened up the Microkorg case in search of the spot to extract the audio signal. Using the service manual as reference I was able to solder a signal cable in the right location.