Marantz 2230 to subwoofer ?

I have  a Marantz 2230  receiver made in 1973 anyone know how I would hook up a self powered subwoofer to it. 

from the speaker terminals
You would need to find a sub with 'high level' input to connect to the speaker terminals.
Wow, that's the only way?
If your reciever has pre-outs (I believe the 2230 does) then you can run rca cable to line in on your sub. 

I notice a warning, your receiver MUST have common grounded outputs, or it will damaged if connected to the subwoofers speaker- level inputs! 
I hate to assume but I assume the Marantz 2230 has common grounded outputs??? Anyone have a clue on this one before I start playing around??
thanks again for everyone's help!!