Hi @tronke ,
Have you proved that there is output from the receiver to the sub woofer?
Try connecting a DMM (digital multimeter) set to 20V AC scale to the receiver's SW output and check for a reading when playing audio which would use the SW
It most probably will be less than 20V but just selected that as a safe starting point.
You could also try low frequency response headphones and check if you hear it at the receiver as well, but the meter is more conclusive if you're not sure about the headphones' response to low frequencies.
Just a thought.