Receiver impedance level

Typically the impedance switch selects different taps on the secondary of the power transformer. This results in changes to the DC voltage available to the output stage, which reduces its heat dissipation at the expense of maximum power output.

The purpose of the impedance switch relates to bench testing with pure tones. Any real world source of music has 4-10 times or more times less heat-producing capability than a pure tone with the same peak level.

The primary purpose of the impedance switch is to protect the AVR's amplifier output stage from destruction...
Todays AVRs are typically built under very tight component material cost budgets, so the brand strives to save $..
If one is running low impedence/sensitivity full-range loudspeakers and being driven to higher SPL levels, it is easy for the AVR's power supply and amplifier output stage to be stressed. And amplifier stage failure is one of the major reasons AVRs are sent in for warranty repair...

Its basic function is too limit the current available, by reducing the power supply to a calculated max voltage output. Also when set to a lower impedence position such as 4 or 6 Ohms, the dynamic range capability of the amplifier is reduced as the amplifier stage has max output (voltage & current) without this limitation. So for the highest sonic performance it is best to leave it in the 8 Ohms position.

Just my $0.02...

Even if the speakers are 6ohm?

yes. it's not a tube amp that requires (or at least performs optimally if) the output transformer properly matches the speakers. what people are saying is that with a solid state amplification device there is no need to match output impedance to the speaker impedance. and that's not what those switches do. They just keep the amp from making as much power as it otherwise would, mostly to keep regulatory authorities happy so it won't get too hot under any conditions. If you listen to music and watch movies (rather than playing pure sine waves at max output) you are quite unlikely to encounter a heat problem. Unless you have very inefficient speakers that you sit very far from and listen very loud.

you can do what the switch does just by turning down the volume control, in other words. except with the switch on the lower impedance setting you'll get more distortion at higher sound levels, which you don't really want to recreate anyway.

Regardless of the brand/model loudspeaker connected, leave the switch in the 8 Ohms position..

Just my $0.02...

Ok thanks