the first thing you need to find out the subwoofer use left, right or both channels by add subwoofer pin and remove one of the headphone pin
speaker-test -c 4 -t wav -D hw:1,0
do you hear sound from subwoofer when "Rear left" or "Rear Right" is playing
the first thing you need to find out the subwoofer use left, right or both channels by add subwoofer pin and remove one of the headphone pin
speaker-test -c 4 -t wav -D hw:1,0
do you hear sound from subwoofer when "Rear left" or "Rear Right" is playing