Best multi-purpose subwoofer

I don't use subs with my main listening system as my speakers are 3 db down at 20 hz, but I do augment bass for video use and run a pair of the Hsu VTF15H Mk 2 for home theatre use, and agree that they are excellent.  Hard to get  more bang for your buck than that.

Strongly suggest that those who can afford it get a matched pair rather than a single.

Duke/Audio Kinesis,

     Thank you for your unexpected but very informative and interesting contributions to this thread.
     I think many have the somewhat mistaken belief that attaining good in-room bass response is simply a matter of using 1 or 2 subs of a particular, or 'right', brand and model in their systems.  
     My experience is that 1 or 2 good subs can achieve good bass response at one specific sweet spot in most rooms but this results in inconsistent bass response throughout the remainder of the room, heard at various specific spots in the room as bass peaks, nulls and even bass absence.  For those unconcerned with good bass response being restricted to a specific sweet spot, I think 1 or 2 subs properly located can be a satisfying solution.
     The distributed bass array concept of 4 subs properly positioned definitely has the advantage of producing very good bass response throughout the entire room for those that require it. 
     However, I've noticed other benefits to the bass quality that were unexpected that I'm theorizing are only possible due to there being 4 subs dispersed throughout the room and their cumulative affect. 
     My best description is the bass in my room became more agile or flexible in the sense it can faithfully reproduce whatever bass is dictated by the source material, whether it's music or HT.  It can be finely detailed on music that allows for the easy identifying of the different bass instruments being played, following the separate bass lines and also the small changes in pitch, tone and volume of each instrument.  The bass can also be powerful and delivered with sudden impact which helps convey the large dynamics of live music and the low frequency effects on HT.  
     My main point is that a distributed bass array system, whether the Swarm or a custom setup, has more benefits than just good bass response throughout the entire room, it also elevates the quality of that bass to state of the art.  I've never been able to achieve this level of bass quality in my system with 1 or 2 subs.

Tim

Thank you Tim and Duke, and everyone else who has go graciously taken the time out to write and educate. There is a lot to this subject just like any other piece of equipment in the audio chain, and to me that is where the most fun is being an audiophile. 

In regards to subwoofers there is no debate that multiple subs are better, but to choose one that matches with your room acoustics, and your R/L speakers is equally important. I believe for HT it makes the task a little easy, but for dedicated 2 channel critical listening it gets tough. How well does the subwoofer pair with your R/L speakers cross overs will depend on how well and smoothly it rolls off, bringing the mids and low to life. For my Von Schweikerts VR4 that hit 20Hz comfortably and smoothly, it’s really difficult to pair a good subwoofer. The only way is to audition - but it’s really a hassle to ship the product back every time. Again this is a journey and definitely I am  just now in the beginning stages (currently own a NHT SW1 8 inch sub powered by a SA1 mono amplifier). I have owned the sub since 1998 and it’s time to upgrade - mostly for 2 channel listening. 

The new SVS SB3000 has some great refinements and flexibility. Buy a set, critical imo for 2 channel. They are very compact, fast and outperform their price point.