Recommendation for Moderate Stereo Upgrade?

Just got an '08 GT and looking to "flesh out" the sound the stereo makes.

I'm no tuner or audiophile; I like the treble and midrange sounds, but am just looking for a bit more oomph from the bass.  Should I just look to add a subwoofer?  Would doing that compromise my interior space at all?  Anyone with experience here?

Thanks!