I just received my Aperion Speakers (4-522 D's and S12 Powered Sub, still awaitning 522D VAC) and have a question surrounding speaker impedance.  The speakers are attached to my receiver - Yamaha RXV-2500, and within the GUI automated setup menu  the receiver asks to select a speaker impedance (6 or 8 ohms).  My question is as follows:

In the Aperion speaker specifications located in the user manual it states that the speakers have a nominal impedance of 6-8ohms...with this being said at what level  should I set my receiver's speaker impedance, 6 vs 8 ohms?  Thanks in advance for your support.