Officially, only the Audioengine S8 subwoofer is compatible.
But any subwoofer with a chinch input will work, as the Audioengine A2+ has a chinch output.
◾Dimensions: 15.24 x 10.16 x 13.33 (cm)
Source: http://technikblog.ch/2014/07/testbericht-audioengine-a2-kompakte-aktivlautsprecher-mit-grossem-klang/
LG