Can I just add an active subwoofer to my stereo?

Hi there, 

I was wondering (since my speakers do not provide the rich sound that I would like with a little bit more bass) if I could connect an active subwoofer of any kind to my amp if I connect it like in the picture. It's called a tri-wire setup, but I am not sure what sub to buy, or if this even is possible with my amp.
I have a stereo amp with 2 speakers (duh) and on the amp it says "9ohm impedance only". But is this also with active subs, since they are amped themselves? 
I want to add a small one, I'm not an extreme basshead. just want to add a bit more sound. 

Thanks

I'm not familiar with the tri-wire setup but for adding an active sub it's irrelevant. The signal going to the active amp should be the same as going to your other amplifier, these are low level inputs. The speakers are connected to high level outputs, so these can only be connected to special high level inputs, which are not on every amplifier.  So either split the cable going to your low level inputs, or look for a :`tape out` on your current amplifier and connect those low level outputs to your low level input on the sub amplifier. 

Cubo