I recently participated in a Podcast focusing on subwoofer performance with Home Entertainment magazine's Editor-in-Chief Geoff Morrison, Director of (loudspeaker company) Revel Products, Kevin Voecks, and Brent Butterworth, a freelance writer for a number of magazines including Sound & Vision.
Voecks immediately zeroed in on the importance of placement, which is absolutely crucial to getting the best out of any sub. I'd go so far as to say an average sub, perfectly placed and set up, will outperform a great sub that's been carelessly placed and set up. That's because the sub's interaction with room acoustics can play havoc with sound, all too frequently resulting in boomy, uneven bass.
True, you can get away with that to a degree if all you're trying to do is add oomph to movie soundtracks, but a muddy-sounding subwoofer will muddle the sound of music. Some call it "one-note bass," and subwoofers that sound like that bug me.
You don't get to hear much from me on the Podcast for quite awhile, so I'll cut to the chase: Buy the biggest subwoofer you can stand. Or to put it another way, there's no way a 1-foot cube sub will ever outperform a substantially larger, but less expensive, sub (say 15 inches tall by 15 inches wide by 20 inches deep or larger) when it comes to pitch definition and bass clarity. Just want boom? The mini subs will be fine. Want musical bass? Get a bigger sub.
For more from me, check out my own subwoofer setup guide.