Originally Posted by tcb-player
This seems confusing and contradictory, but maybe I'm just reading it wrong. You say the speakers will always be 8 ohms, then say they do not have a fixed impedance
Seems to me the highest impedance setting will yield the Lowest current, not the Highest. If you had an amp Without settings, it would draw more current at lower impedances, not the other way around.
I think the goal is to pick the setting that most closely matches the speaker impedance, although the actual speaker impedance may vary wildly over frequency, so you have to trust that the "nominal" rating is representative.
I was generalizing about them always being 8 ohm. That was probably a bad idea to keep that there after the note of varying impedances.
Anyways, the impedance setting in your receiver does not actually affect your impedance. That is determined by the physical properties of your speakers. Instead it affects a limiter of some sort on the power supply that reduces the maximum current the system can output. The lower you set it, the more it limits the current, for supposed "safety". Plenty of amps DO lack impedance settings, including most pro amps and stand alone home amps. In fact, it seems the nicer or more powerful the amp, the rarer these switches become.
The reason being, with the smaller amps in receivers, there is a concern they can't supply the needed current (or dissipate the heat from it) to 4 or 6 ohm speakers all the time, and could overheat if driven hard. The amp of course will go into protect instead of being damaged, but which would annoy the average customer more: A menu setting that makes the device "safer", or an AVR that randomly shuts down for reasons they don't understand? Of course, it's the first one. They wouldn't know it cuts down on the amp's dynamic power while not actually making anything or anyone safer, including the amp itself.
Sound engineers and audiophiles/hobbyists would know why current limiting is bad, and why amps go into protect, and would prefer not to have these switches on their amps. They would want them to have full power all the time, and shut themselves off it comes to that. Hence the reason pro amps and standalone home amps generally don't have impedance switches.
Now that you know, you don't need one either, so go ahead and set it to the highest impedance choice (least current limiting)