Are you using these are computer speakers or home theater speakers in full room? Audioengine makes their own subwoofer, not exactly a whale-beacher but should integrated with no fuss.
If you're at a desk, I would look into a sealed unit. Keeps the size down, and in nearfield listening spaces they offer more then enough output to compliment the speakers (unless you only watch ware of the worlds, then i would go for something larger).
What models are you currently considering?
SheepStar