Possible ways are to get "Y" splitters and add them at the pre outs, with one pair going to the amp, and the other to the sub. Another (worse idea) is most subs have 'in' and 'out' RCAs to route the signal from the pre via the sub then from the sub to the amp.
Another is that some subs can accept power amp level signals, and you can biwire the sub into the amp output.
All of these ways leave 'something to be desired'.
Good luck
Another is that some subs can accept power amp level signals, and you can biwire the sub into the amp output.
All of these ways leave 'something to be desired'.
Good luck