I did what you are trying to do by purchasing a $13usd amp and hooking it up to the Sub out on the A/V receiver. And my old passive Subwoofer.
Works well for the money. I used an old IBM laptop power adapter. Or you can buy one recommended on Amazon.
I registered soI can post this in case it can help you.
Hopefully you can find a similar amp in India.
AOSHIKE 8-25V 100W TPA3116 Subwoofer Digital Power Amplifier Board TPA3116D2 Amplifiers NE5532 subwoofer Plate Amplifier
https://www.amazon.com/dp/B01N0PXEN4/