Subwoofer


A couple of days ago I was talking to a dealer and he said that all speakers benefit from adding a subwoofer. What's are your thoughts? 
ricred1
For me ( born perfectionist) audio is comparing and testing.....over and over again. This part never stops. In almost 18 years of time I have done thousands of tests in sound&vision. Only to find those products who give the best results. Because inferior products don't add anything to the world of audio.

For me audio is not about good, but only about the best. Because this way you will get only clients with a big smile on their face. 

In almost 18 years of time I learned that over 95% of all products in audio are not worth it's money. 
There are many subs with 15" drivers, all sounding a little different from each other. Which gives "a more natural sound"? The one you own, of course ;-) ! The 12" and 15" Rythmik subs, both with aluminum coned woofers, sound exactly the same---not just very similar, but identical. The 15" just producers more output, due to it's larger cone area, longer cone travel, stronger motor, higher-watt amplifier, and larger sealed enclosure. But the sound characteristic? Indistinguishable from the 12". Rythmik designer/owner Brian Ding says so, as do Rythmik owners having both 12" and 15" models. But hey, feel free to "believe" whatever you want.
My closed box Miller&Kreisel subs are paper. They can sound shockingly natural with percussion. I strongly felt I got on the right path with CB and paper after having a bunch of good quality OB subs with a range of different cone materials. Rythmik's are my next subs of choice, again with CB and paper. They left me as if not more impressed than the MK's. It wasn't my listening environment and I'm anxiously awaiting to hear what they do in my own space.        
Í would never advice a subwoofer which is not sealed. Each single part to improve the timing is an advantage in accuracy and integration. When you use subwoofers with a 15 inch driver, you get a more natural sound. Also with more authority.

@ricred1 : I will send you a personal message this weekend. I had to find out how I can send a personal message.

To add to Duke’s very informed post above, Danny Richie’s GR Research OB subs are the "fastest" I’ve ever heard. Danny chose paper as the cone material for the drivers he designed for his subs for a few reasons: He finds paper to provide greater resolution than aluminum at small signal levels, the result of paper’s lower mass and energy storage in comparison to aluminum; he finds paper to provide more natural timbre than aluminum; and paper-coned woofers can be used to a higher x/o frequency, his OB sub being usable up to 300Hz. Danny offers the same woofer with an aluminum cone (marketed through Rythmik Audio), but recommends the paper version for highest sound quality.

The "fast" characteristic of a sub is not, as Duke just said, the result of it starting quickly (bass frequencies are relatively slow), but rather of it stopping quickly (the OB sub has been described as "stopping on a dime"). That results in the sub not filling in the spaces between musical material with noise, from both the cone itself as well as the sub cabinet---self-induced resonance. The OB nature of the sub also lessens the room’s ability to add it’s own noise---the "boom" subs are notorious for creating. An OB sub doesn’t "load" the room the way sealed and ported subs do, exciting fewer room modes.

Danny was already offering OB subs when he learned of fellow-Texas resident Brian Ding of Rythmik, who had developed and patented a new servo-feedback woofer design. Danny proposed they work together at designing a woofer optimized for Open Baffle use and employing Brian’s servo-feedback circuitry. The resulting sub is very special---the world’s only Open Baffle-Dipole/Servo-Feedback sub! Brian himself offers an extensive line of aluminum-coned Servo-Feedback subs through Rythmik Audio, in 8", dual 8", 12", dual 12", 15", dual 15", and soon-to-be-available 18", some of them in sealed models only, some in ported only, and some in both. Very "fast" sounding subs is what Rythmik Audio is known for (Sterling Sound in NYC, famous for their superior-sounding mastering, has six of the sealed 15" in their monitor systems), and GR Research has a few DIY sub models (sealed in addition to the afore-mentioned OB), as well as loudspeakers. There is no longer any reason or need to settle for "slow", bloated, boomy subs!

Just for the record, I am skeptical of the notion that cone material makes an audibly significant difference in the low bass region, as long as the cone is stiff enough to behave as a piston. I don’t doubt that a person could hear a difference between different subs and that difference could seem to correlate with cone material, but cone material would not be the actual cause of the difference.

As for "speed", the ear has incredibly poor time-domain resolution in the bass region. We cannot even detect the presence of bass energy from less than one wavelength, and we must hear several wavelengths before we can hear pitch. So whatever it is we are perceiving as "speed", it is actually something other than the leading edge of a low frequency signal. We will come to exactly what it is in a minute.

What the ear is extremely good at, in the bass region, is hearing differences in SPL. Look at a set of equal-loudness curves:

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/imgsou/eqlou.gif

See how the curves are close together below 100 Hz? What that means is, a small change in SPL is perceived as a much larger change in loudness. So a 3 dB change at 30 Hz can SOUND LIKE as much of a difference as a 10 dB change at 2 kHz!

(Incidentally the ear’s exaggerated sensitivity to small changes in SPL at low frequencies is one of the reasons why it takes a long time to get the gain dialed in properly on a subwoofer system.)

Low frequency speakers in rooms have a minimum-phase characteristic, which means that the time-domain response tracks the frequency response. So when we have a peak in the frequency response, we also have ringing at that frequency (conversely if we fix the peak, we fix the ringing). At low frequencies it is this MODAL RINGING ASSOCIATED WITH FREQUENCY RESPONSE PEAKS that sounds "slow"! This happens on the TRAILING EDGE of a low-frequency waveform, where our perception is well-equipped to detect what we interpret as "speed" (because we are good at hearing loudness), NOT on the LEADING EDGE!!

Recall how it takes us several cycles to hear the pitch of bass energy. By the time we hear bass, the energy has reflected off of room surfaces numerous times. So PERCEPTUALLY, we cannot hear the leading edge of a bass waveform, nor can we separate the subwoofer from the room. They are a SYSTEM, as far as human hearing perception goes. So imo it makes sense to design subwoofers that work with the room instead of against it. If the in-room frequency response is smooth, then we don’t have some frequencies decaying significantly more slowly than others, which contributes to the perception of "speed".

Now it is common knowledge that sealed boxes sound "faster" than vented boxes. This is explained by their in-room frequency response trends. A vented box tends to be flat down to a much lower frequency, and then roll off rapidly below the tuning frequency. A sealed box starts rolling off higher up, but more gently. Once we factor in "room gain" from boundary reinforcement, the vented box typically has a net rising in-room response as we go down in frequency. Couple this with room-induced peaks plus the ear’s significantly increased sensitivity to small SPL changes in the bass region, and vented boxes do indeed tend to sound boomy. The more gentle rolloff of a low-Q sealed box has a much better synergy with room gain from boundary reinforcement in most cases.

"Typical" room gain from boundary reinforcement is ballpark +3 dB per octave below 100 Hz. A low-Q sealed box has a rolloff rate that approximates -6 dB per octave, so the synergy there is good. Imo it still leaves room for improvement, but that would be another topic for another day.

Controlled blind listening tests have shown that group delay on the order of what we might expect from a well-designed high-quality vented box is inaudible on program material, and just barely detectable on test tones. But the difference in frequency response is highly audible. In the real world the time and frequency domains are linked in the bass region, but it is our perception of loudness in the frequency domain that dominates our perception of "speed". When a kickdrum goes from full-on to very quiet very quickly because there is no modal ringing (because the in-room response is smooth), that sounds very FAST and TIGHT. (The frequencies that give the kickdrum impact are largely above the subwoofer range, so we use the gain, frequency, and phase controls to optimize that blend.)

I’m not saying there would be no audible difference between a sealed box and a vented box equalized to have the same in-room frequency response, but I am saying that any difference would have nothing to do with the leading edge of the waveform.

Anyway my point is, the differences in the perceived "speed" of subwoofers are due to factors other than cone material or the leading edge of a bass waveform. Perceived "speed" in the bass region is more closely related to the in-room frequency response than to anything else (which is in turn linked to modal ringing or lack thereof), so that is the thing we should focus on first. I’m not saying that’s the only thing that matters, but I am saying it matters far more than the subwoofer cone material or the leading edge of a low-frequency waveform, neither of which human hearing is well-equipped to hear.

Duke

bo1972,
On another subject, I'm still looking for your comments on the Monitor Audio PL500 speakers?
Some of the leading speaker manufacturers such as Scan Speak.employ paper, I suspect for good reason. Name calling is childish, I’m not here to debate with the likes of that. Or narrow mindedness. As far as material goes, no one can ever prove which is superior because it comes down to a personal presence, as do many things in this hobby. Enjoy your shootouts.
When you still believe paper is the way to go for subwoofers, you would be a fool. To a few Velodyne owners I have proven that their subwoofers are too slow. 

You need to make audio understandable for many people. Shootouts and explanations why the sound is what you hear is the essential key to bring people to a higher level in sound quality.

Even with a 4000 watt amp, a paper driver is still slow in response. Doesn't make sense. 

You need to look further to focus on the best response/timing possible. To create this each part needs to be used to the max. 


Many brands still use drivers with material what is too slow. Even EQ will not solve this limitation.

I also use Ultra Stillpoints, which make a huge improvement in timing and accuracy. Same about using a 100% silver interconnect.

At the end each part you can use to improve response/timing are essential to create a stunning integration.

Because most subwoofers and the way people use them are still too slow for a optimum level of integration.

In the past subwoofers were used more often for multi-channel, but.....these days you have the option even for stereo use to create a superior level compared than any system without a subwoofer.

In 2005 I thought this was not possible. But now I can say; It is possible. This makes stereo so much more appealing to listen to music.
Regarding how "fast" a sub woofer responds. I do not buy into many of those points. I believe it has more to do with driver motor assembly and the amplifiers dampening factor than cone material and overall weight. I often prefer paper and infused paper for its tonal qualities, never much considered its weight. But have had subs that employ poly and aluminum cone materials that did very well at reproducing natural tones and textures. I never detected any being inferior in terms of pace.

There are way too many factors at play, more than I care to try listing. To sync the subs to every set of mains would be major a feat in itself I would imagine. I would bet most if not all to be off if even by a millisecond or two. Such a minute difference that you wont be able to detect any difference without the use of the multi thousand dollar measuring devices anyway.

Does this thread mean we're not far from subwoofers moving from the "Home Theater" category and into the "Speaker" Category here on Audiogon?  ;-)

In the past I have sold many Rel subwoofers. For surround I liked them, but for stereo use they were too slow. Because when we did tests in response and timing it was easy to find the limitations. For a perfectionist they are never acceptable.

In the last few years I have auditioned the latest Rel subwoofers as well, they are still not fast in response enough.

Material use is essential for the best response. You should use light and fast materials. Like ceramics and aluminium. That is why I don’t understand why they use the material they use. For Velodyne the same story.

The Velodyne subwoofers were the first which I auditioned who could create a sound without problems with the acoustics of the room.

This was an important part what was not that good in the past.

But still you had other limitations, these limitations were:

- Not a perfect integration with the speakers. It often felt if instruments and voices became more blurry and less focussed during shootout.

I have done a lot of research in room acoustic measurment and I also created a different way to measure a subwoofer. With results were we could only dream of 10 years ago.

At the end it is all about timing. First of all you need subwoofers with a response fast as a rocket. Second you need a professional room acoustic measurement system. And you need the best profesional lasers you can get. And you need to understand music.

This last part is forgotten in audio many times. This had become a serious problem. When you ask simple questions to people who give an audio demo and they don’t know anything about music. Audio becomes useless.

A new approach of measurment, professional microphone and professional lasers has created Stealth Low frequency.

It fully integrates with the speakers. But the main difference is that the energy of instruments is exactly were it the energy should be. When I give  a demo you see their reaction that it cannot be possible. Even when the subwoofer is over 3 metres away from the energy.

The funny thing is that years ago people here at audiogon told me that I a am a liar. The people who auditoned it I have told this story often.

They had to laugh and said: I understand their reaction, because they never auditioned it themselves. For me and my clients it has become quite normal.

And yesss stereo with a subwoofer always will win from any speaker set without a subwoofer. Even when you use a speakerset of 500.000 dollar with starts at below 16hz it will not win.

Because when you use stealth low frequency each part you judge sound for gets better. And even at low volume you can create a much more realistic low freq. Because even a speakerset of 500.000 dollar needs to be played at extreme volumes before they can reach 16hz.

I never thought that I would sell stereo with subwoofers. Now in 2016 over 80% of all my stereo clients use a subwoofer.

When we visit shows ( even Highend Munic) often demos with subwoofers are so limited. Because most don’t understand music and audio.

Bass blossoms all around, it submerges you, yet not to overwhelm. In turn it highlights contrasting frequencies. That I believe is what gives a greater sense of depth and realism to the sound stage. 

Thank you for this.  This is probably the best explanation yet I have heard on why a sub seems to improve overall experience.  
My subs are very well integrated into my system, the room treatments have quelled the spurious room resonances that detracted from the bass, and the system sounds great.

So last night I fired up the system with the pre-amp output to the subs turned off, the system sounded great that way, then I turned the pre-amp output to the subs on and it was like the "Wizard of Oz", when Dorothy approached the kingdom and the film suddenly was in color!
That’s my take on why people rule subs out. After having much difficulty integrating one their first attempts having a less than great experience they become frustrated give up and rule them out. Ill suspect an issue in the room before a $1500 rel. It comes to mind when I see comments suggesting subs sound "bad", or subs just aren’t for me. That would suggest only certain personality types are suited to subs. That’s silly. I’m convinced they just haven’t "cracked the code" and had subs and the room properly dialed in. Or might be too overzealous with gains. Once you have its a whole new ear opening experience. Bass blossoms all around, it submerges you, yet not to overwhelm. In turn it highlights contrasting frequencies. That I believe is what gives a greater sense of depth and realism to the sound stage. Vision is a sense that reacts positively to contrasting colors making opposing colors appear more vivid than they actually are, standing them further out. Hearing being a sense why would it be any different? As I sit and listen that’s what seems to be going on, and it makes perfect sense to me ;) Full size mains can achieve the same effect quite well, but you can still make it bigger, better.

"Duke, does your opinion change if I am only interested in good bass at a single location in the room?"

If you're going to be equalizing for that single location, I can only think of one theoretical disadvantage of using just one sub:  If you have a significant dip at that location, you could conceivably overtax your sub trying to fill it in with EQ.  That being said, dips are subjectively much more benign than peaks. 

I have been told by someone who tried it both ways that the distributed multisub system sounded more natural in the sweet spot than a single equalized ubersub, but that could have been due to other factors.  And my sample size (in this case, one) is too small to draw any conclusions from. 

Duke


So while adding a single subwoofer will offer the benefit of improved bass extension, adding multiple smaller subs and spreading them around intelligently will additionally result in smoother bass throughout the room.
Duke, does your opinion change if I am only interested in good bass at a single location in the room?  In my case, I have a dedicated, albeit small listening room where I listen from a fixed location.  
I guess it’s fair to say, "all speakers benefit from using a good subwoofer." So, in general, will good speakers with dual subs sound better than very good speakers without subs?
"still most subwoofers own a slow response." So is there a commonly published measurement or some design criteria that separate the winners from losers in your opinion? Besides Duke's innovative approach mentioned above, was there a "game-changer" in subwoofer design that sets more recent apart from older ones?

When you say "your clients" you imply industry affiliation. You should disclose that in signature of your posts. 
Cheers,

Spencer
In the past I said: a subwoofer for stereo use is useless. But....faster subwoofers, using them till 120hz and my own developed Stealth low frequency made me think totally different about subwoofers for stereo use.

But.....still most subwoofers own a slow response. This means still most subwoofers are useless. Even in 2016 there are many subwoofers who create more difficulties than advantages.

About 80% of my clients buy speakers with a subwoofer. Because I have proven that I can create a higher endresult with a subwoofer compared to a situation without a subwoofer.


A normal home listening room will impose huge peaks and dips on the output of a subwoofer or pair of speakers.

As we increase the number of intelligently distributed bass sources in a room, we significantly reduce the magnitude of those peaks and dips. The only way multiple subs could fail to make the net response smoother would be if they all had identical in-room response, and the distribution ensures that nothing even close to that will happen.

So while adding a single subwoofer will offer the benefit of improved bass extension, adding multiple smaller subs and spreading them around intelligently will additionally result in smoother bass throughout the room. By way of anecdotal evidence, it seems like most Maggie and Quad owners who add a single sub go back to using their speakers without subs within two or three months. But it seems like most Maggie and Quad owners who add a pair of subs keep them in their system. Those few who have tried more than two subs never go back, as far as I know.

I have multiple customers who report +/- 3 dB in-room from 20 Hz up through the bass region without any EQ from using four small subs. And smooth bass is "fast" bass, subjectively speaking, because where there are peaks in the bass region, there is ringing. Smooth the peaks and the ringing has been addressed.

While the output of a single subwoofer can be equalized to be very smooth at a single location, or almost as smooth within a small area, such equalization will inevitably make the response worse elsewhere in the room. Because a distributed multisub system results in much less spatial variation within the room, any need for EQ is probably correcting a global (room-wide) problem rather than a local one. So EQ is arguably rendered even more effective by a distributed multisub system, assuming it’s even needed.

A distributed multisub system results in a greater number of smaller peaks and dips in the in-room response. The subjective improvement is often greater than the raw numbers would lead you to believe, and here’s why: The ear/brain system tends to "average out" peaks and dips that are within about 1/4 octave of one another, so when we have a lot of smaller peaks and dips bunched up together, they start to behave as a continuum (as far as the ear/brain system is concerned). This is what happens higher up in frequency (those hashy-looking room-reflection peaks and dips are there, but they’re so close together that we don’t hear them discretely), and this is also why large rooms have subjectively better bass than small rooms. A distributed multisub system can make a small room behave like a much larger room at low frequencies.

Here is what UCLA mathematics professor, concert violinist, and respected audio reviewer Robert E. Greene had to say about a distributed multisub system: "Makes even the most magnificent of one-point subwoofers into dinosaurs, something grandly impressive in their time, but their time is over."

So imo there is arguably significant room for improvement beyond the addition of a single good sub.

Duke

dealer/manufacturer

I added an REL S5 to my Tidal Contriva Diacera-SE speakers a few months ago and it has made a significant improvement.
You need to understand that a speaker can give it's lowest frequency when you play at loud volumes. When you play at normal volumes, the speaker does not go that low.

Subwoofers go deeper at lower volumes, this is a big advantage. And now comes the main part, you can point out low frequencies from 80hz. 

When you use a subwoofer with an extreme fast driver ( ceramics/alluminium) you can use it till 140hz with ease. This is including acoustic measurment. 

The influence becomes even bigger. It can make instrument and voices more palpable. You even hear more details when you compare the same music without a subwoofer.

Even stage depth and width is expanding with a subwoofer when you are able to use it the right way.

Most subwoofers are to slow in response, so you can't use them over 100hz. In these situations you are better of without a subwoofer.

But.....in 2016 it is possible to create a higher level in sound wiith a subwoofer than without. And yes we are talking only about stereo use.
All speakers benefit from subs in varying degrees. Even speakers with capable sub bass modules at there bases. I agree with subs enhancing and increasing sound stage and all frequencies adding detail and putting more air between instruments. And the more room modes you can activate the more evenly distributed bass and flatter response becomes. But don’t expect a sub or multiple subs to perform miracles if room acoustics are inhospitable, thats at least half the challenge of acquiring well tuned bass thats tight and natural. Another point worth mentioning again is trying to practice some resistance its much too easy to introduce too much bass. Often listeners will go overboard on gains then wonder why they aren’t happy with end results. Just because its there doesn’t mean you have to use it. Less gain is usually more in terms of over all sound quality. Your less likely to start burying other frequencies or pushing the listening environments capabilities or carrying the sub nearer to its distortion levels where it starts becoming more obvious. One more reason I prefer using more than one sub and as many as 4...5..6?? If you have the space to go crazy I say do it.
danvignau,
I'll repeat what stringreen said, " You're not extending the low end, but making mids and treble better." That's has been my experience with my current set-up. My soundstage has opened up, mids and treble has better detail.
If you want helicopters in your listening/TV room, a sub is wonderful. I have experimented for years and prefer no sum except when required for extra bass impact. All salesmen say that anything they sell helps.
the difficult part of subwoofers is not messing up the upper bass, lower midrange...
The beauty of a subwoofer is in it's ability to be placed in the best location for your room . Your mains are restricted in a fixed immovable location due to a triangle listening setup .
bondmanp,
I think what he was saying is, no matter what the listed frequency response of a speaker is, in his experience all speakers benefit from using a sub.
The question is, will spending an equivalent amount on better speakers yield similar results as adding a sub?

All speakers? That’s a blanket statement which implies the person making the statement has heard all speakers with and without subwoofers.


That said, IME, I have generally preferred systems which properly use a subwoofer than those that do not. However, a poorly implemented subwoofer can do more harm than good.


In my own system, I use a pair of Vandersteen 2Wq subs with the battery biased MHP5 crossovers. I will never let go of these high-value, highly underrated subwoofers, no mater which speakers I will own. My mains extend into the low 30Hz range on their own, but when dialed-in properly, the Vandy subs are simply stunning in their ability to fill out the lowest octave, add depth and width to the soundstage, and produce a wonderful foundation for the rest of the range. YMMV.

My "main" speakers are -3dB around mid-40s with a fairly shallow rolloff.  I added subs below 25hz (72dB / oct) using an analog electronic crossover built by an audio designer who knows what they're doing.  Steep cutoff below 10hz.  I built a pair of 12 cu ft cabinets, each housing 1 x 18" Eminence Kilomax driver.  150 watt mono amp per sub.  Corner placement behind the mains. 

Crossover could be higher with a strong likelihood they would interfere with the mains (soundstage, image, details). 

Nice DIY subs.  But they do need a bit of extra power to compensate for their natural rolloff.  (Crossover has an additional response curve to counter the natural driver rolloff in that enclosure.) 

I try really hard to keep from "dropping the needle" when playing vinyl. 
tboooe,
I swear to you adding subs has "opened things up and expanded the soundstage." I don't know why, but it's easy to hear.
What has been difficult is blending them perfectly!

I sure hope so!  I am going to pull the trigger on a REL T-5i sub.  I think in my modest room and style of music  this sub should work for me.  

Where do you guys suggest I set the crossover point?   My speakers start to roll off at 72hz and its -6db point is 43hz.

I am on my third setup with sub woofers.  I find you must have patience to get them properly integrated into your music system.  But once done, they add so much to the sound of the system, they take the great sound from the main speakers and make them sound awesome.

One of my audio "mentors" used to tell me that the best sub woofer was one that didn't sound like it was there, unless of course, you are a person whose pants hang down, exposing your undies.

This last go around was the most difficult integration of all, as the room had all sorts of resonances that interfered in a big way with the law bass.  It took the installation of 10 bass traps and tuning the crossover frequency, curve and volume using the Room EQ Wizard software and calibration microphone to get it sounding "just right".  Now that it does, it just sounds awesome!
Amphion Argon 2s, JBL PSW 1000.
Couldn't get the sub dialed in for months. I Couldn't find the right place where the sound changed from hear to feel, base was either boomy or lacking, No external crossover, just trying to get the sub to roll in where the speakers rolled off.

Then My wife and I attended a Bach organ concert at a big, stone church (Univ of Chicago, Rockefeller Memorial Chapel). After hearing (and feeling!) what "live" sounded like it was just another hour of tweaking and all was good!
I do buy into the idea that subs aren't just for improving the bass but the overall experience of the music.  I played around tonight with an AR H6 sub (100W, 8" driver, ported) from our very modest home theatre that was on its way to Goodwill.  Stuck it midway between a pair of Prelude Plus speakers facing right at the listening position (not orthodox sub positioning at all) and approximately on the same "front" plane as the Preludes.  Found some old JPS Labs speaker cables and hooked the sub to mono block amps via speaker input terminals.  Started listening with level and crossover dialed all the way back; gradually increased things.  Found a combination that seemed to blend well...almost couldn't tell the sub was on but for the effect on music I knew well.  Dunno if it was power of suggestion/self-delusion but I'd swear imaging was clearer and sound stage bigger than without the sub.  A fun evening.  Definitely thinking now how nice things will sound with a pair of sealed, better quality subs that are properly positioned.
tboooe,
I swear to you adding subs has "opened things up and expanded the soundstage." I don't know why, but it's easy to hear.
What has been difficult is blending them perfectly!
tboooe.....you'll be happy.  Its not necessary to get the most expensive ones you can find.  Even moderately priced subs help and will not destroy your very fine Sonus Fabers.
Very timely thread!  I am thinking about adding a sub to my Sonus Faber Guarneri Evolution monitors.  These have a -6db response at around 43hz and starts to roll off at 72hz.  My room is small (10x11, though the back wall is only 3' high and opens up to my family room) and I do not listen at high volumes.  The sub I am looking at has a -6db response of 32hz.  I am hoping that adding a sub has the effects that Stingreen mentioned above.  I have also read that adding a sub will help to "open things up" and "expand the soundstage" though I have no idea how.

 You're not extending the low end, but making mids and treble better....also you change the acoustics of the room by evening out the "bumpy" low end.  

Highly recommended as long as you do the management properly.  Look at the MiniDSP 2 x 4
I have learned that without proper phase integration with the mains, the sound will suffer big time. When the sub and speakers are in time,the whole system improves across the board... When out of phase,bass becomes slow and lacking in dynamics and musicality.
Stringreen 3-18-2016 6:39 p.m.
I explained it and some of you still don’t get it. You’re not extending the low end, but making mids and treble better....
Assuming, of course, that a high pass filter of some sort is inserted somewhere in the path to the main speakers. And assuming that the benefit of keeping the deep bass out of the main speakers is not outweighed by the sonic side effects of that high pass filter and its associated interfaces.

As is usual in audio, a tradeoff is involved. And I wouldn’t by any means expect the net result of that tradeoff to always be the same among different systems, rooms, recordings, and listeners.

Regards,
-- Al

What stringreen says is true for most speakers, none more so than planars. When you add a pair of subs to Magneplanar/Eminent Technology or ESLs, you remove the very low frequencies from the panels, decreasing their displacement and the resulting distortion (planars are very inefficient movers of air), resulting in better upper bass and lower-mids on up into the midrange itself. You also remove the low-frequencies in the signal going to the speaker's amp, decreasing IT'S distortion, and leaving more available power for the mids and highs.

With the quality of musical subs available these days (Rythmik, GR Research---their OB/Dipole especially!, SVS, Funk, Seaton, REL, JL Audio, Vandersteen) there is no reason not to.

It depends!!!!   If the sub is quick/fast, as with the better sealed subs, then yes definitely. If the sub is sluggish, as with many ported subs, then the system can become a 'muddy mess'.

Either way, however, it requires some patience and experimentation to get everything to come together.

I personally use a sub with all of my systems. Even with speakers that are essentially flat to 20 hz.

I have experimented with several different crossover settings and have found that 90 hz works best in my room.

stringreen,
I agree that adding subs has significantly improved the mids and treble
My experience with a sub is that even if your speakers have good low end response because the sub is going to be placed in a different position in the room, you will get more pact and generally a broader soundstage 
I explained it and some of you still don't get it.  You're not extending the low end, but making mids and treble better....also you change the acoustics of the room by evening out the "bumpy" low end.  Add the sub...or 2...or 3 
Getting bass right is vastly more difficult than getting higher frequencies right. You simply have to move more air and deal with the interference issues attendant to the omnidirectional nature of low frequency information. Powered subs are purpose built for the first job and allow placement flexibility for the second issue.

I think subwoofers are almost always just a better mousetrap

YMMV