That will do it. Even headphones jack work as before. Of course this is only scheme, You don't necessary need to place in-built speakers outside Red line is left speaker signal, green is right speaker signal, black is ground, and mixed colors cable is one cable with amplified-lowpassed signal from both channels.

Alternatively, You can use only one lowpass-filter, but then You need to place it AFTER amplifier. This way, you amplify all spectrum, then lowpass it - approach with first lowpassing, then amplifying resulted output gives better quality. Ho ever, with such subwoo and speakers, You'll probably don't even notice difference. Still, with both methods, You probably can break window's glasses in 100 meter diameter, on full volume

Placing adjustable resistor just before subwoo allow to calibrate bass volume - as built-in speakers aren't amplified (don't try it, amplifier will blow Your speakers, they're doing their best now), this is probably must-have if You decide to do it as on my scheme.