I have a bluetooth speaker, the Soundboks 2, to which I want to add an additional subwoofer. The Soundboks already has good bass response given that it has two 10" cones, but it is battery powered and lacks the power output that a wall-powered sub could provide. What kind of sound equipment would I need/how would I go about this? The Soundboks can receive input from bluetooth or aux.
Speakers like these are self-contained units, not meant to be added or add to existing blah-blah. Only possibility I see is, if you are driving it via AUX, can use a Y-cable and send that signal to the sub. Via BT, u gotta open up the unit and... if u have to ask.
As suggested above splitting the source that connects to the aux input is the easiest way.
If you have to use bluetooth you would open the cabinet and connect a speaker to line level adapter to the terminals of one of the woofers. That would provide a line level output that you could connect to a powered subwoofer. An RCA jack could be installed to make disconnecting the sub easy. Not electrically complicated but you would void the warranty.