What could I do to make the bass' sound better without changing the car too much (to not lose the warranty)?
get a bigger sub


as you expand the space you need more power to fill it. would your home stereo be sufficient at the super bowl? they use massive power and broadcast systems because of the amount of air they have to move.
Your hatchback was considerably less volume than the limo (lucky for some!!) and the smaller (?) sub was enough. there is probably some equation like square the volume and cube the power or something. Any pro-audio PA guys out there?

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Murray