Finally, I made it to client's site and back. None of these codes did work and ONKYO itself wasn't aware of anything useful. Looks like they stripped the firmware really down...
Last possible way is to use the front panel LEDs and use them as feedback to the RFX9600 Extender, but client is not willing to pay for this. Other amps would be cheaper anyway...
If you don't want to get better you stop being good.