There are a number of terrific small subwoofers on the market, but all of the best subs are big. The little ones can certainly make bass, but the quality and quantity of the larger subs' deep bass is considerably better. You can literally feel the difference -- deep bass is as much felt as it is heard.
I remember the impact a big sub made when I reviewed the Outlaw LFM-1 (now upgraded to the LFM-1 Plus, $549). The LFM-1 weighs 58 pounds and measures 21.75 inches tall, 15 inches wide, and a whopping 22 inches deep. It had a 12-inch woofer and a 325-watt internal amplifier. Smaller subs have to work a lot harder to make room-filling bass; the LFM-1's 12-inch woofer and larger cabinet made more bass with greater ease. That's key: a big woofer in a small box won't cut it; cabinet size plays a crucial role in the story. If you've never heard what a big sub can do, it's hard to put into words how the very foundation of the sound changes. The LFM-1 not only made a lot of bass, it made the speakers sound better, more dynamically alive and powerful. If you already have a decent set of home theater or stereo speakers, even fairly large ones, the Outlaw LFM-1 can take the sound to another level.
The Outlaw sounds spectacular with a 12-inch woofer, so what would a 15-inch woofer bring to the party? Well, if it's Hsu Research's 123-pound VTF-15H ($879) 15-inch subwoofer, you'll feel the sound through your feet! This is a good time to bring up room size. Making credible bass in small, under 300-square-foot rooms is easy, but folks lucky enough to have 800 square feet or larger rooms should seriously consider getting a big subwoofer, or more than one subwoofer. The VTF-15H would be a good start; it measures 18 by 25 by 26 inches.
I've heard the big Hsu sub at various hi-fi shows, and it's truly awesome, but still nowhere near state of the art. The critics response to the Paradigm Reference Signature Sub 2 ($9,000) have been nothing less than raves. The 230-pound, six-sided, 4,500-watt beast has six 10-inch woofers. Having drivers deployed on three sides of the cabinet made it possible for the engineers to keep the size down somewhat, but it's still 24.5 by 23.75 by 22.3 inches. The six woofers together weigh 150 pounds! Used in a large room, the Sub 2 can generate a truly visceral experience commensurate with the image quality of a big plasma or video projection system. Home Theater magazine's Shane Buettner was wowed not only by the Sub 2's brute strength, but he sensed more bass' texture and detail. Buettner said, "I had no idea what the hell was going on in those lower-frequency ranges," before he heard the Sub 2. Great subs aren't just about power -- they can sound delicate and nuanced when the recording's bass is delicate and nuanced. Lesser subs miss that sort of detail.
All three of these subs can radically transform the sound of a home theater for the better, but careful subwoofer setup and room placement are essential to hear the subs at their best.