Help with AVR & speaker crossover frequencies

There is no clear rule. If you were making detailed measurements in an anechoic measurement chamber, it would depend on which of your speakers has a better response in the 70-80 Hz range, the sub or the others.

But you aren't making detailed measurements, and any actual room in your home is going to be far from ideal. You just want the best sound possible to your ears. So test it. You probably won't hear any difference at all. If you do hear a difference, imaginary or real, set it the way you prefer. It isn't going to make a big difference.

Speakers may have 8/6/4 ohm impedance, probably in that order of how common it is. This says little about the volume the speakers produce.

First, how much volume is produced for a given amount of power depends on the speaker's efficiency. It varies widely. A lot of high quality speakers tend to be very inefficient because they fine-tune the sound by trimming away unwanted bits, sort of like trying to make a perfect paper circle with scissors by trimming off imperfect bits. You end up with a much smaller circle than you started with in order to get it as round as possible.

Second, this is just the nominal impedance, because it actually varies widely across the frequency spectrum, and different speaker designs may have very different impedance curves. Normally they specify something close to the lowest value to make sure that you aren't being misled into over-driving your amp, but the average impedance may be much higher. I've seen speakers rated as "8 ohms" or "4 ohms" that had impedance curves that could easily have been rated either way depending on what the manufacturer thought was significant.

In any case, you will normally turn the volume up to a comfortable level, averaging way below the maximum power the amp can produce, regardless of the speaker impedance. All things being equal, an amp can normally produce more power into a lower impedance speaker, but the difference in volume that produces would be hard to even notice because of the non-linear way your ear hears volume vs. power.

Most amps don't have any sort of "ideal" impedance setting. If the power output is within their rated range, there should be no audible effect from driving speakers of different impedance.