This all assumes you want to connect and active sub not a passive one.
Well since that unit was never designed for a sub, I would recommend trying the tape out or line out if you have one.
Tape out or line out is a line level copy if incoming signal send out for recording on a tape recorder. This should be a level a sub can accept.
Settings like bass enhancement may not work since you do not want to record those to tapes anyway.
This is the best I can come up with aside of running a splitter to send the incoming signal to the sub as well(again line level).
In a real pinch of you have NO line out, you can also use the schematic here to get line level from the amp outputs(the speakers can still be connected). I am not sure if it would have any advantage over using a splitter from the line level out on your device to the sub and speakers.
If you use the tape out of the receiver then since that does not change with playback volume the bass level will not change as you turn the receiver volume up and down. Better to use a speaker to line level adapter such as this http/www.ebay.com/itm/like/330593999790?lpid=82
which will let you set the level of bass relative to the speakers once. You could also build one as nukemaster mentions.