Randy, if I was doing this I'd want to do it with the least amount of effort and expense. I'd probably get a short F/F cable such as this to plug into the wall plate and then connect an F/RCA adaptor such as this to the other end to plug into the sub.

Since you apparently already have the coax and F connectors, you can make up the cables yourself and just buy some of the adaptors.

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Enjoy the music, not the equipment.