inputs/outputs, you run the "mains" speaker wires from receiver to sub, then the sub's speaker(high-level) outputs to the mains. I don't know if you can disable high-pass for the mains in this regard, as I've never used hi-level before, and do not have them on my sub. It would slightly surprise me, but who knows!
however, if this is for 2-channel music, for accurate response, you want to have the mains as "small". Now, that doesn't automatically mean you have to set the x-over at 100hz or something, but it can be at or above the natural roll-off of your towers, even if low as 30hz.
If this is for HT, you definitely want to use the low-level (LFE) output, and feel free to use mains as large if they can handle it. This is another story entirely.
The sub out of my marantz sr6200 has no signal. My sr6200 has main outs. Can I use the front main outs to go into my psb subsonic 3 and back to the 6200. I guess I just need to know if the outputs of the sub is fullrange.