Your AVR has a passive sub output.. Even if you add a bigger passive sub, the power will be limited by the AVR output..
But, if you were to add an active sub to your AVR, then you will need to choose a subwoofer with high level inputs & outputs.. Pic below for example..
You will need to set subwoofer as 'NO' in your AVR settings, which will redirect the bass to the front left & right speaker..You can then use the speaker output terminals on AVR to connect to speaker terminals on the subwoofer, and to connect speakers from subwoofer, with a crossover on the subwoofer.. Might be clumsy with wires, but will work until you upgrade your AVR..
What is your budget for subwoofer?..