Did you replace the speakers? I doubt I need a sub now...
I changed the two front speakers and the rear speakers in the side panel (because it's a convertible). The swap of the front speakers made a big change, the rear speakers weren't that big of an improvement.
There are still the two (stock) subwoofers in the door, but they don't have that power of the subbox - also the bass now goes really deeeeeeeeep. In some song's I now can feel(!) the bass at points, where nothing was before. (O.K., not my best English sentence, but I hope you'll know what I like to say).
It's not necessary to have a sub in the trunk - but what is really necessary? I feel better now - and if you install it yourself, it sounds much better
My car: 2011 V6 convertible Premium, Performance white, automatic
Mods: Pioneer&Kicker speaker, Airraid CAI, SCT-Tunes, painted mirror covers, 3.73 gears, FRPP LCA's, One-Touch-Convertible-Module, Fog lights