Hi Alex/Anonymous.

I have an OnkyoTX-DS797 and the owners manual states:
"If the impedances of all speakers are between 6 and 16 ohms, select "6 ohms". If the impedance of even one speaker is between 4 and 6 ohms, then select "4 ohms"."

I would assume that Onkyo receivers in the same family will probably function the same. Hope this helps. :)

b_man