I interpreted the OP as asking about connecting to a subwoofer amplifier using the L/R line inputs, assuming his subwoofer amplifier doesn't have speaker-level inputs, since many do not have them. My own also do not have them. In this circumstance, you need something to convert speaker-level outputs from your main amp to line-level inputs of your subwoofer amplifier. The Iso-Max device is one option, and it looks like there are cheaper options made for the car audio market as well (a la the Amazon link I provided).
I also have a janky old cable designed for this purpose by Ed Schilling of Hornshoppe which I believe is just a set of RCA cables with the RCA connectors cut off on one end and some resistors installed in-line before the RCA plugs on the other end. Not sure about that design, though. And he might also be doing something "creative" with grounding too. Anyway, it has worked in a number of setups, but often doesn't provide sufficient range of motion on the subwoofer's gain settings, I assume due to the resistor values chosen. Ed described it as a cable to help out situations where you couldn't get enough gain from your subwoofer amp for satisfying synergy with high efficiency main speakers.