Evening

Good question. The stereo input is probably used to route the left and right channels.
(if your woofer has an RCA output). Normally a mono connection is sufficient.
Or you can try it out by plugging a remaining cinch cable into the input of your subwoofer and connecting the other end (inner conductor) with your finger.
end (inner conductor) with your finger. If there is hum at the red and black inputs, I would use a Y-cable.

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