You need a sub-woofer with speaker level input:
http://www.gammaelectronics.xyz/images/stereo-review_sept_1995_subwoofer_f3.jpg
OR
a sub-woofer with speaker level input and output:
https://images.crutchfieldonline.com/ImageBank/v20160824170200/ImageHandler/scale/978/978/core/learn...
http://www.gammaelectronics.xyz/images/stereo-review_sept_1995_subwoofer_f3.jpg
OR
a sub-woofer with speaker level input and output:
https://images.crutchfieldonline.com/ImageBank/v20160824170200/ImageHandler/scale/978/978/core/learn...