the easiest way to get more loudness is a bigger box with a bigger port turned 5hz or so higher. also... resin the inside of the box, use sections of PVC pipe to round the corners of a slot port and the box itself. flare your port opening. brace more to reduce resonance, seal the mounting hole better and play with the boxes location in the hatch/rear/trunk.
Sound deadening gives you more apparent loudness by blocking out outside sounds and reducing energy wasting panel resonance.
New subs that are better suited for your listening habits or more efficient with the amount of power you have give you more loudness with the same power.
you can upgrade your sub amp.
you can run your current amp at a lower impedance (if it can take it)
an upgraded electrical system (big 3, bigger power run wire, high output battery, second battery and HO alternator) give your amp more electricity to work with and reduce voltage drop which allows your amp to play louder.
and... i dunno, i'm sure i'm forgetting plenty of stuff, but theres lots of thinks you can do to a daily driver to get louder...