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.
http/www.epanorama.net/circuits/speaker_to_line.html