How do you set the crossover on the S8?

The S8s crossover is a variable low-pass filter. This means it variably adjusts how high the S8 will play. Unless the subwoofer is pretty far away, you can usually leave the phase switch set to 0°.  When setting up a new S8, I like to start with the crossover all the way counter clockwise and, with some music I'm familiar with playing, first get the volume "about" right.  After that try making very small adjustments clockwise on the crossover, evaluating each small change from where you'll be listening from most often.  Once it starts sounding like you can localize where the sub is coming from, back it off until you can't again.  You may also need to lower the volume a touch after the crossover is dialed in, or perhaps even while you're dialing it in, depending on the acoustics of the room you're in, and your own preference on how present you'd like the sub to sound.

Audioengine powered speakers have fixed passive crossovers designed specifically for them; this simply means that they are designed to play the frequency range they produce, so it's not necessary to alter this when adding a subwoofer.  Adding a subwoofer, such as the S8, will pick up where the speakers leave off, extending the low end response.