Did the OP get any good, clear options? I am thinking a simple sub swap wouldn't fix it, as I think the amp just isn't doing its job. Anyone found a way to adjust the amp gain to the sub? Or anyone tried replacing the sub only? I have seen someone mention the dealer applied upgraded firmware for the AMP? Thanks! I have no bass and am just sick of it! I can't even feel the sub box vibrating!
There are no cures without going off "the deep end" as I call it. You can't fix what the stock setup is, and isn't, without attacking the issue at the core of the problem. Its a bad "premium" system. I made my Durango absolutely jaw-dropping to hear (not talking about loud, but sound quality/accuracy/tonality) but it took a processor, amps, tons of deadening, etc. and while it was fun, its not like I can suggest everyone go do that. I can ASSIST though, for those seeking. The hard part is finding a processor with surround capabilities. Those are rare right now, for some unknown reason.
Now, I had this in one of my threads before, but this may need some fleshing out. I'll focus for now on the subwoofer side of things
Here's what the stock subwoofer does, the teal line is without the sub connected, dark line is with. So the sub is responsible for the difference.
Closeup of that action
So the sub DOES do something, but because the truck's parts back there by the sub are not properly deadened, the whole thing resonates and "rings" at low frequencies. What you see here is a waterfall graphic, a measurement of response over time. So the back of the wall in the graphic is the initial response (in this case, of the LF woofer/tweeter and the sub together), and notice that in the lower frequencies there's response still going in the 35hz range. This is bad. It mostly indicates that the speaker itself is ringing (box too small is my guess) and things around it are too.
There's more I can show you, but its a lot of data. Basically, I think you could succeed in making a better subwoofer solution, but it fixes the bottom octave of a deeper problem, if you're seeking a highly accurate, detailed, honest to the music system. Then again, if you're just after more bass, yeah throw some subs at it. Probably grab signal off the front woofer door output and you're going to get what should be a decent response.