Audioengine's speakers, particularly the A5+, offer rich, lively tone and great imaging. Whether you're using them for near-field listening in front of your computer or as the main speakers in a living room or dorm room, they're probably the only speakers you'll need.
But with that said, sometimes it's nice to have a bit of extra low end to really fatten things up. I find this especially true for watching movies— The Matrix and The Lord of the Rings really come alive with an extended low end. That's where a good subwoofer comes in. The best is one that's matched to the speakers in question, which is why the Audioengine AS8 works so well with the A5+. It's a compact subwoofer, with an 8" down-firing driver, and controls for volume, crossover, and phase. It's more than up to the task of picking up the low end—in fact, in our shop we have to keep it tuned pretty low because the sub is so powerful.
I recommend this without reservation.