Thanks for the detailed explanation.

So, was I also wrong on my very first post too?
Is that mean that I was double wrong??


Cannot be, but then, that would not be the first time.

Yep, you can connect so many different type of subs in so many different ways, it's just mind boggling sometimes.

No wonder, one gets confused.
Hey Soundman, it's your lucky day man!
Because your vintage Marantz Stereo receiver has both Pre Out/Main In, you can easily connect a sub through these.
* You just need two pair of analog stereo interconnects.
#1. On your Marantz, where it says Pre Out, just connect one pair of interconnect there (Left & Right channel), and then connect the other end to your subwoofer Low Level IN (RCA In jacks). If your sub has only Mono In, just use a Y adaptor (2 females & 1 male).
#2. On your Marantz, where it says Main In, just connect the other pair of interconnect there (L & R channel), and then connect the other end to your subwoofer Low Level Out (RCA Out jacks). If your sub has only Mono Out, just use a Y adaptor (2 females & 1 male).
*** If your sub does not have Low Level Out (RCA jack(s) Out), use some speaker cables from the sub High Level Out (Speaker binding posts) to the respective Left and Right speaker binding posts of your two front speakers.
-> You don't need that interconnect any more, from the Main In of your Marantz. And you don't connect any speaker cables from the back of your Marantz either. Set the Low Pass Filter to where about you thing is best (eg. if your two main speakers have good bass to about 60hz, try a Low Pass Filter at about 80hz). And just adjust the Gain (Volume Level), from the knob on your sub, to match equally the Volume level of your Marantz.
That might take several tries to balance properly, just be patient and experiment with some CDs containing great bass passages in their recordings.
Acoustic bass would be best.
[OK, now I'm ready for the critics.]
Cheers,
Bob