I have the older 4.2SC and also thought the bass was lacking.

What I did was to add sound deadening material to the front doors and subwoofer enclosure (subwoofer in this car is beside the amps in the boot).

This made a huge improvement over the standard system - note that I did not make any (other) changes to the system. In the past the bass would sound tinny and the subwoofer would rattle - now there is this good bass. Unfortunately it does not increase loudness but as I said the bass is much better.

See thread “My other covid-19 project” on this forum