2.1 systems generally use powered subs, not passive subs.
For use with higher-quality bookshelf speakers, you can send the stereo line level signal to a 2-channel amp and pass the high-level speaker lines through your sub. I do this with Belkin AirPlay 2 receivers and it works well.
For use with higher-quality bookshelf speakers, you can send the stereo line level signal to a 2-channel amp and pass the high-level speaker lines through your sub. I do this with Belkin AirPlay 2 receivers and it works well.
Not all subs have high-level passthrough though.