BRW, welcome. The first point is that you're paying too much attention to speaker impedance ratings. Speakers vary widely in their impedance as they play various frequencies; often the rated impedance is approached less than half the time.
Specifically as to the impedance setting on the receiver, the higher default setting, which may be labeled as 8 Ohms or 6 Ohms, should always be left in effect, regardless of the rated impedance of the speakers. Changing the setting to the lower impedance number is specified in manuals because of safety requirements of the UL(Underwriters Laboratories)relating to overheating(of the receiver)with low impedance loads. It does this by reducing the voltage output of the receiver's power supply section, which following Ohm's Law results in a reduction in current and power. Tests have shown that the maximum power capability is cut roughly in half at the lower impedance setting.
So, the maximum capability of the receiver is available at the default higher impedance setting, and the speakers would be in greater danger of being damaged by receiver clipping because of less clean power being available at the lower setting, which in no way can improve the sound of the speaker.
Never change the 8 ohm setting.
Enjoy the music, not the equipment.