When you use the RCA input of the A2, the 3.5mm input actually acts as a line-out. You can use this line-out to feed any active subwoofer. I've tried this when I used to have A2s and it works.
You can also think of getting the Audioengine AS8 subwoofer, which will most likely go very well with the A2s. In this case, your source input will go into the AS8, and the AS8 has a line out that will feed your A2s.