Hi Thanwu,
Odd indeed. No-one at Axiom should be telling you to set your Yamaha at a 6-ohm setting. I'll have to do some internal detective work!
This has been a continuing issue with various brands of AV receivers for years because of the 4-ohm M80ti towers, which many brands will not drive without overheating, current limiting or protection-circuitry shut-down. As Bren pointed out, the presence of an impedance-selector switch suggests that the AV receiver has limiting circuitry that will severely limit the power output to lower impedances to avoid liability and overheating issues. Leave the switch at the 8-ohm setting to maximize the receiver's potential output into 6-ohm and 8-ohm loads.
Axiom has been advising this for years now.
Regards,
