R-40PM - Subwoofer Output
You can connect any powered subwoofer the the Klipsch R-40PM using a single ended RCA cable. Connect one end to the "SUB OUT" port on the back of the R-40PM, and the other into the "LFE" port on your powered subwoofer.
Adjust the gain knob on the subwoofer until you are satisfied with the output. You can also adjust the line level volume to the sub using the included remote control.