Speaker level inputs on the subwoofer would be the best way to go. A lot of good subwoofers have speaker level inputs.
Sub woofer naive question day
I am a geezer and have had decent audio equipment for better than fifty years. But have never owned a sub woofer. What is the proper way to implement the speaker into a system with an integrated amp that does not have a specific sub tap? The amp has a line out and a pre out.