How do I make my existing Audioengine Subwoofer 'wireless'?

You can do this using powered speakers like our A2+s, HD3s, HD4s, A5+s, and HD6s.  For this, you'll connect the W3 sender to the RCA output on the left speaker, using a 3.5mm stereo-mini to RCA "Y" cable.  Then connect the W3 Receiver to the input on your S8.  You can either use a 3.5mm stereo-mini to stereo-mini cable for this, or another 3.5mm stereo-mini to RCA "Y" cable; either will work fine.  Both W3 units will need to be powered from a USB port.  You can use USB AC power adapters, or if the powered speaker you're using is our A5+ Classic model, you can use the USB port on it's rear panel for the W3 sender. The same would apply to the S8 subwoofer if you have our current 'Gen II' model that has a USB port on the rear panel.