Audioengine makes some accessories, including the S8 subwoofer and a couple of wireless adapters (the W1 and W2) for its speaker line. The subwoofer would obviously allow you to get significantly more bass and richer sound, but the downside is that the sub is actually pretty pricey ($350) and bringing in that extra speaker somewhat diminishes the sleek, minimalist effect these speakers offer (for Apple systems like the iMac, the white version of the 2 series tends to match up better than the black does).
Luckily, the appeal of these speakers is that you do without the sub. For their size, Audioengine 2s deliver deep, tight bass, and offer excellent detail and relatively big sound, even though they're so compact. True, the Audioengine 5s deliver a richer, fuller experience, but for many, the step-up model will just seem too bulky to leave sitting on a desk (they truly are bookshelf speakers with a more industrial, monitor flair to them).
As we said, we liked what we heard from these guys and so did Steve Guttenberg, who writes the Audiophiliac blog and reviews home theater speakers for CNET. He, too, praised the bass and noted that "the midrange and treble refinement are exceptional." You can crank the volume on the 2s but he, like this reviewer, thought they sounded better at more moderate volume levels.
Because they deliver such good sound for their size, we were tempted to give the Audioengines Series 2 speakers an Editors' Choice award, but ultimately we felt the higher-end 5s were more worthy, mainly because they offer even better sound and such niceties as a USB port for charging devices like iPods and iPhones, an extra 3.5mm input on the top of the speaker, and even a power outlet on the back of the speaker for plugging in additional audio sources, such as the Apple Airport Express. (Alas, the 5s don't include a remote either, which is too bad.)
In the final analysis, if you're looking for a set of compact, good-looking PC speakers that deliver great sound for their size, look no further. We should also mention that the Audioengine 2s perform better than most of the iPod speaker docks in this price range. Of course, it helps that you can separate the speakers to your liking, which allows for much better stereo imaging.