I have 13 RDX (Non-Tech AWD) for about 4 weeks and it sounds like the subwoofer in the trunk just lost its kick though seems still working very weak. When changing the subwoofer volume in the setting, you don't hear the difference and normal music play comes with much less bass than used to be. Just wondering whether any user has experienced same as I don't see the posting of similar issue on this site.

 

This is nothing but an educated guess.
I'm pretty sure the sub on the base RDX doesn't have any type of crossover to restrict the sub to only play low frequency. It's probably being fed the same signal that goes to the other speakers, which is why you hear the full range music from it instead of a thump.
When I was a teen I would have chopped into the harness and installed a passive crossover which would have restricted the signal to only play lows. It's a cheap way to "fix" inadequate sound using the stock system.
Now that I'm older, I wouldn't try it because I like my warranty in place. In addition, I simply went deaf when I was a teen who modified stereos to thump, so there not a whole lot of incentive to modify it.