The optical output on your akai is "spdif", not "adat". So it only sends 2 channels of audio. Spdif is the same output you get on the back of cd players, and is stereo.
This is totally different from Adat lightpipe which has 8 channels.
You can connect spdif to a digital mixer, but you will probably need to change it from optical to coaxial, since that is what just about all digital mixers need.
If you want to send 8 channels to a digital mixer, you will need to buy a special "ADAT" add-on card from Akai for your sampler. Then you will be able to send and receive using the ADAT lightpipe format.
I have an ADAT card for my S6000 and it is connected to my PC. With two Adat outputs, I can transfer 16 channels at once (into a motu 2408). It works very nicely.