Subwoofer connection

Do you already have a subwoofer?
If not, look for a subwoofer that has speaker level connections. 
A popular brand such as REL Acoustics Provide such a connection. 

I do have subs w/amps. Just trying to figure out how to connect to receiver withought RCA sub output. Can one connect through a pre out? I’m not sure if I even want a sub for this thing as it’s driving everything I’m throwing at it like Klipsch, Cerwin, and Bose with descent base. Thank you!

@gkelly  is correct, in that a subwoofer with speaker level connections would be the way to go. However, because vintage receivers do not work with banana or spade speaker connectors, speaker level sub woofer connection is not a straightforward process. I doubt you can fit two wires into the small gauge holes, so you may need to connect the sub or subs to the speaker "B" inputs and run A+B output.

Whatever you do, I would first check with REL or whatever sub manufacturer to decide to go with, to make sure it will work without damaging either component.

Pre out should work fine ...