Advice for wiring sub

I have a nice old Aragon 4004MKII and I need to add a subwoofer. This is a 2 channel stereo amp and there is no sub out or any other connection.
Is there a way to wire the sub even though there are only two channels?
Get a powered sub that can take its signal from the amp output. REL can do this , I think Hsu also as well as others. Check specs to see if the sub accepts high level as opposed to line level input. It will derive a mono signal from the two stereo channels. You can drive two subs of this type from a stereo amp, I do.
Hi Stan,

Thanks for the response. I found some piggyback plugs that allow me to add a second set of speaker wire to the original output.
Interesting suggestion regarding the second sub. I have my eye on a older Phase Linear 3000 that might be a good sub amp.
Powered subs do not require an amp and you can't use an ordinary amp driven off speaker outputs without an adapter to reduce the level of the output. This is built into some powered subs, not all.