With amplifiers where no pre-out or dedicated subwoofer output is fitted, you will need to use a subwoofer with 'high level' (speaker connection) input to connect to the speaker outputs of your amplifier. On an amp with A/B speaker switching you can use the speaker B terminals for this purpose.
If your amplifier only has a single set of speaker outputs, you may need to use a speaker switching box to generate a second speaker output to connect your subwoofer. An active subwoofer does not generate any load when connected over high level terminals so the impedance calculation is relevant only to your speakers.
Please note that a passive subwoofer will require an additional amplifier.