DIY subwoofer amp

I happened to get my hands on a Boston acoustics Soundware XS subwoofer with the plate amplifier gone missing. So am trying to see if I can build a plate amplifier using this class D amp from Aliexpress. TPA3251 High Power HiFi Digital Amplifier Board with High Power Stereo 220WX2 Bridge 350W AP2202

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This is meant to be a stereo amp with an option to bridge to connect to a subwoofer. When I connect the LFE from my Yamaha AV receiver to one of the RCA's on the amp board; it works perfectly. However, I am relying on the Yamaha AV receiver to cut off all the high frequency. If I were to connect my TV audio out then there is nothing to filter out all the high frequencies. Hence, thinking of adding a pre-amp board like this.

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The low frequency output from this preamp board should connect to the amp at the XH2.54 3p audio input terminal shown by the red arrow.

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I need some help to decide how I make this work.
1) The amp already has a potentiometer that acts as an on/off switch and volume control. If I were to add the preamp board should I remove this potentiometer on the amp and use only the volume control on the pre-amp board?
2) How do I switch on the amp without this potentiometer switch? There are 2 wires separate from the rest indicated by the yellow arrow which looks like they might be connecting the on/off jumper and if I short them the amp should remain switched on? Is there any way of knowing this for sure using a multimeter ?
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Any help would be much appreciated. And please pardon my lack of knowledge. I have no background in electronics.
Many thanks.