Install a subwoofer for deeper bass from the audio system along with a separate subwoofer amplifier.
There are plenty of third-party products in a wide range of sizes and capacities. The rear trunk footwell is likely the best spot for adding a subwoofer, assuming no rear-facing seats are installed. A decent speaker amplifier combo will start at $300 plus installation costs, and easily top $2000 for a high-end system.
The Subwoofer location can just about be anywhere. It's sound can't be easily localized by the human ear. It's one reason why audio systems are 5.1 or 7.1, the ".1" is the subwoofer. It doesn't need to be in stereo. So placement in the front or rear will make little difference to the perceived sound quality.
I got the Reuse audio upgrade for my model S and it significantly improved the clarity. The upgrade consisted of a replacement for the sub with a sealed 10" sub and a new amp plus replacing the two tweeters and adding a third tweeter mounted behind the rear-view mirror. The improvement over the sound studio was significant. I think Tesla could provide a similar upgrade for less cost if they so desired.