Subwoofer output should be in a Pre-out section(RCA). That receiver is quiet old, I don't know what kind of support it had for subwoofers. Does it Have Dolby Digital? If it only has pro-logic, it might be a high level output (speaker wire style).
Marantz's website is quite pitiful, and lacks the manual for that receiver. If I had to venture a guess though, it would probably have to be one of these (see below). Any pre-out should suffice as long as you're hooking up the subwoofer with a pre-out. Otherwise it will need to be hooked up using the outputs for the front L and R speaker.
I put a star underneath the one that I think is most likely it.
SheepStar