Convert it to rca ?
No conversion necessary,just fit a single rca plug to each end.
Its pain with coax cable but absolutely doable,just buy decent wireable plugs.
You only need single subwoofer output on the reciever to either the left or right rca inputs on the sub (not both)
If youre currently running both plugs from the amp to the sub its completely unnecessary,thats a 7.2 amp, those outputs are for 2 subwoofers not a single connection to one - Id imagine only one output is even active unless youve purposefully enabled twin subs in the amp settings menu.
https
/techsupport.cambridgeaudio.com/hc/en-us/articles/200551522-Why-are-there-twin-RCA-inputs-on-my-subwoofer-