Hi @Ayan Haldar,
The problem might not be in the sub woofer. Here is a link which details a similar but slightly different problem with a X210. There the problem turned out to be in the cable between the volume control and the sub woofer. Again with your problem you don't know yet whether it is low output on the left channel from the 'volume control' or from the output of the amp in the sub woofer.
This link has good pictures of the boards in both the sub woofer and the volume control and also explains what was done to find the problem as there is no wiring diagram available.
If you have little or no experience working on electronics initially it is quite OK, with the power unplugged and the unit is also disconnected from the PC, to carefully open the unit(s) and visually check the circuit boards and see if there is anything obviously wrong in either the sub woofer and the control unit, such as faulty capacitors (domed or exploded), heat stressed resistors and other components, dry solder joints etc. It would be unwise to probe the boards with the power connected as one slip could have catastrophic consequences and you would be worse of. There is also the very real danger of working in close proximity of lethal voltages, in the sub woofer at least.
You could take pictures of the boards, as was done in the link and post them here to see if anyone here can spot something that you may have missed. Adding images to an existing question
If this sounds all to daunting then I suggest that if you want to have the unit repaired that you take it to a reputable, professional electronics repair service and ask for a quote. If you do decide on this option at least that way they will be responsible if anything goes wrong during the fault diagnosis and repair of the unit.