My main problem with the sub is that it is only felt when you are in the trunk area. It does not resonate throughout the entire car. I was looking into replacing the sub, so I took it to a shop. The installer did not take the panel off completely because other panels have to be removed but there is not much space behind the sub because that's right next to the wheel well. Also, replacing the sub without an amplifier is pointless. No matter how powerful the new sub, the most you will notice is about a 5 to an optimistic 10 percent difference if you are running it directly from the factory wiring. Problem with adding an amp is that it messes with the Active Noise Cancelling, which consists of mics placed around the car. You can disconnect it from the back of the head unit but one of the mics that the ANC uses is the one used for the Hands Free Link, so it will not function optimally. And you will then need to replace the speakers and get an amp for those so they do not get over powered by the sub. And that point you might as well swap out the head unit for one that allows you to control the sub and the amps, which requires a new dash kit lol.
In a very detailed way I am basically saying, if you're not willing to change it all its not worth changing lol. Had to do this in the accord coupe. First, I started with the sub (pioneer 12 in), a 2 channel amp, and 4 pioneer 4 ways. I had no real control over anything due to the stock head unit so I went back. I then got 2 pioneer tweeters to replace the stock ones, a 4 channel amp for the speakers, a capacitor, and ended up getting a pioneer double din head unit to be able to control it all with ease. Also had to buy an adapter to still be able to use the steering wheel controls and a bypass switch was placed in a hidden spot to allow the head unit to play movies while driving (I know its illegal... don't care lol). You do lose the factory clock and Handsfree Link by replacing the radio. I can take some pics of the set up if you want since they're basically the same. Just gotta wait till 2mro when there's light lol.