The rating is the nominal impedance and varieswith frequency; it is not a single number. When you have a lower impedance number, e.g. 4 ohms, then more power is required, P=v^2/r =I *V =R*I^2 Typically AVRsare current limites and sometimes get a "Protect" fault when the current is too high when the volumeis turned up on lower ohm speakers. Some receivers that can handle 4 ohms are therefore designated as high current AVRs (receivers). The Onkyo handles 4 ohms very well, inn fact as good or better than most AVRs.
Bottom Line I wouldn't worry about it! your Onkyo should do fine. If you have pre-outs, you could always add an external amp later if you do run into "Protect" issues.
Peace and Good Sound to All!
Forest Man