Hi cocheex
That's a very nice system you have there, I think you're going to love it. First, you have to go to the advanced set up menu and make your selections.
I'd set all the speakers to SMALL, and cross over at 80 Hz. Make sure you enable your subwoofer "SUB YES/NO."
Onkyo has various surround/DSP options accessible from the remote. I think the default is 2 channel stereo, so go through the other choices while standing next to one of the surround speakers and you should find a number of options which enable your center, surround and sub.
Good luck and report back!
