Han, as other replies have indicated, you should only use a lower than 8 ohm setting as a last resort in the highly unlikely circumstance that your receiver would shut down at the 8 ohm setting. Otherwise you'd simply be voluntarily limiting the maximum voltage(and therefore power, which the voltage controls because of Ohm's law)which the receiver could supply. Disregard any advice to the contrary, which may possibly be motivated for legal rather than real technological reasons.

-----------------------------------

Enjoy the music, not the equipment.