Yes! You would just need an amp, like our N22, to supply power to the P4 speakers and connect to the S6.

The S6’s crossover is a variable low-pass filter. This means it variably adjusts how high the S6 will play. Unless the subwoofer is pretty far away, you can usually leave the phase switch set to 0°.  When setting up a new S6, 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 S6, will pick up where the speakers leave off, extending the low end response.

The S6's crossover is a variable low-pass filter, and will not adjust the audio for the RCA output connection. This is a full range line level output.

Yes!  You'd just need a wireless adapter like our W3 Wireless Adapter.

Most Audioengine powered speakers have variable RCA outputs for use with a subwoofer like the S6 (the exceptions being the B2 and the 512). This means that the output of the speakers and the RCA output will go up and down together. So once you dial in the volume of your S6, you should be able to adjust the volume from the speakers, or your source device. Either way the subwoofer and speakers should stay in balance as the volume is raised and lowered.