All good points - and you need to consider all of them.
You also need to remember that there's a lot of grey and not much black-and-white involved.
First, even a 24 dB/octave crossover isn't like a vertical wall.
If you set a crossover at 80 Hz, the small speakers will be down about 27 dB at 40 Hz, and the sub will be down about 27 dB at 160 Hz.
That means that the sub will still be making some output at those frequencies where localization effects occur.
Likewise, the small speakers will still be making some output at 40 Hz (which they, presumably, don't do a good job of).
The frequency chosen is a compromise - and 80 Hz works well for most small speakers and subs.
Sadly, the original idea was that 80 Hz was chosen as an optimum compromise point, and both subs and "satellites" were designed with that in mind.
Recently, with the obsession with tiny speakers, manufacturers are making tiny speakers that don't work well down to 80 Hz.,
so we are forced to sacrifice more by compromising more and raising the crossover frequency into the area where it will be audible.
In the end, the best advice that you can get is "pick the lowest frequency at which your small speakers don't distort or sound strained when you play music with lots of low notes".
There isn't a lot of reason to go much below 80 Hz but, if your auto-system picks 100 Hz or higher , you should consider trying 80 and see what happens.
Originally Posted by BIslander
Why would you use 80Hz as a crossover with speakers that can't handle frequencies that low? 80Hz is the recommended point for THX certified equipment, which leaves out the majority of home AV gear. (Localization can be an issue above 80Hz, of course, but that has nothing to do with the engineers who designed the equipment.) People should configure and calibrate based on their equipment and how it works in their home environments. What do you suppose the engineers did that would make a speaker that struggles with frequencies below 100Hz sound better with the crossover at 80Hz? And why would those same engineers produce processors with variable crossovers if 80Hz is what everyone should use?