Magnifi Sub-woofer - Standalone operation

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I want to use the sub-woofer from my Magnifi soundbar with my audio system, which has much better sounding speakers than the soundbar output, which I am very happy with for watching TV but not for listening to my vinyl record collection. There is a 3.5mm input socket on the back of the Magnifi sub-woofer for cabled connection to the soundbar but but it does not work when I connect it to the SW output of my audio amplifier...... any suggestions as to why this does not work?