From the manual:
RCA LINE IN: Your subwoofer is shipped with a supplied 1/4” Phone-to-RCA-jack adapter already installed in the XLR female input. Connect a shielded coaxial cable, with male RCA plugs on each end, from the A/V receiver or A/V preamp processor’s Subwoofer Output jack to the RCA female jack on the 1/4” Phone-to-RCA adapter.
I would say use the adapter that was supplied and the coax cable, sub cable, you have.
With the sub cable plugged into the sub out of the receiver to the sub in, using the adapter, to the sub.
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