The SU-610 does not have a pre output.
Some active subs does accept speaker level inputs though.
If you can find one, you can use the B speaker connector to drive it.
Another option is to reroute the tape output to the internal pre amp.
It has to be connected after the volume control and before the power amp section.
Any competent tech should be able to sort you out.
Just beware to get a powered sub with fairly high input impedance.
The pre amp section might not appreciate the extra load it has to drive.
I will check the service manual when I get a chance.