For me, it's all about clarity and feel. If the speakers can't produce a nice clear sound, it ruins the music. You have to at least get a 10" or 12" sub to have nice clear bass w/o turning up the bass setting on your headunit (ends up sounding bad that way). The speakers are really for mid-range, not bass.
I have a single 12" in a bandpass box (doesn't take up too much room), an Alpine deck, and upgraded speakers and it's crystal clear. I don't blast my music, but I do have it somewhat loud sometimes depending on how I feel or what is playing.