The S6 is dimensions are 10"H x 8.7"W x 10"D

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.

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.