Start with the system you have. Volume held at its maximum for long periods can shorten the life of speakers and reduce clarity. A system that sounds clean at a sensible level is always a better starting point than one that is simply louder.
Subwoofers can make a dramatic difference by giving music more depth and fullness, but placement, tuning and the rest of the system matter too. Keep speaker grilles clear and clean, and avoid covering the areas that need to move sound into the cabin.
If your music suddenly sounds distorted or muddy, it may be time to have the system checked. Elite Audio can talk through options for a clean upgrade that suits the vehicle and how you listen.