I'm still waiting for delivery of my car so I can't be of much help, but I think the first step is to figure out how much power the amp Nissan put in there is putting out. If the amp is large enough there may be a simple fix by swapping out the speakers for ones which can handle high output without distortion.
If the amp is weak then its a bigger problem. That would mean replacing the amp and the speakers.
I am driving an '07 Z4 (7 days left on the lease) which also has two subs, mounted directly behind the seats. They are also not large, I think they are probably 6.5", but they will blow you out of the car at half volume with zero distortion. The amp is 440 watts, so if the amp is big enough and the speakers are built to take high output, you can get great bass even with small speakers.
Please let us know anything you find out. Thanks.
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