Hello kp,
The optical inputs on a home receiver are AC-3 or optical spdif only, not adat. So optical from your desk won’t work.
If the amp is not providing discreet signals then either your desk is mixing them or the receiver is, and the receiver is the more likely reason. If you want to pursue it, try disconnecting the desk from the receiver then putting one signal into one input of the receiver, with the other five disconnected. This will tell you who’s doing the mixing.
Hugh