Let's get back to our basic subwoofer crossover today.
The purpose of the built in crossover is to make sure the subwoofer doesn't play music higher than a specified frequency; that frequency specified by the user via a low pass filter control. Remembering the term 'low pass' means to pass the low frequencies, this seems simple enough to choose what point you want to have the music to not go through the subwoofer as the frequency goes higher.
For our example we'll continue using 40Hz as the highest the sub is allowed to go. From there, we'll assume it's rolling off at 12dB/octave (remembering an octave is a doubling of frequency). This means our music is 15dB down at 80Hz (-3dB for the set point and -12dB because we're at 80Hz, twice that of 40Hz). Let's also assume our main speaker rolls off at the same rate as our subwoofer, 12dB/octave.
We want to set the subwoofer's highest frequency somewhat below the lowest the main speaker goes. We want to do this because we're expecting - depending - on a bit of overlap, meaning the sub and the main speaker produce some of the same frequencies in music at the point where they crossover - the top of the sub's range, the bottom of the main speaker's range. If both were set at exactly the same frequency, we'd likely get a bump up in sound as the top of the subwoofer will add to the flat response of the main speaker.
So, imagine our main speaker starts rolling off at 60Hz. Setting the subwoofer somewhat lower, at 40Hz, then makes sense because as the one is going up and getting softer in volume, we're getting far enough away from the crossover frequency to minimize the overlap.
Using your main loudspeaker's specifications is a good starting point to set the subwoofer's maximum frequency on the low pass filter. But listening, playing around with the sub's settings will be needed to get the smoothest transition at the point of overlap.