I have an uber awesome amp, why buy a speaker with a built in woofer sub-woofer amp

Most people have enough application for their main speakers, and usually don't have an extra amplifier to drive a sub.   So that's one reason.   The other reason is, by integrating an amplifier, The manufacturer can control  The final product better.  The integrated amplifier in the subwoofer may not be winning any awards per se, but it should be well chisen to get the desired results with that particular sub.  There are manufacturers who make passive subwoofers, but they are definitely less popular. Wilson audio makes two subwoofers,  and I believe both are passive! But be prepared to pay a fortune for them.   In the end I think an amplified subwoofer is not a bad thing despite your concerns over the quality of the amps inside.

Being more convenient having all in one bass unit is an attractive selling feature too. You might have trouble finding passive subs since most of them are plug as complete all in one units. Maybe its time to do-it-yourself subs and buy a pre cut box kit. Some assembly required, or build one from scratch if you have the means to. Then its just a matter of finding a subwoofer driver you like. Dollar for dollar you can get ahead of the game this way and have more performance with some careful planning.

This place has well thought out structurally good and ridged CNC cut flat kits as I was mentioning they just need assembly, ported and sealed versions.

http://www.diysoundgroup.com/sealed-subwoofer-flatpacks.html
http://www.diysoundgroup.com/ported-subwoofers-1.html

A very basic reason it it allows you to use a lower powered amp to run the mid and high frequencies, typically a tube amp often SET.

The built-in sub amp only has to power the low-end frequencies, and is voiced for that specific speaker. It doesn't have to perform across the entire frequency spectrum as your power amp does.
 As a result, your amp now has less of a load to drive. Your amp won’t have to deal with low impedance demands.

I concur- not into this kind of speaker either. Additionally, I do not like the sound of speakers that feature side-fire woofers?

Like JOND said, "it allows you to use a lower powered amp to run the mid and high frequencies, typically a tube amp often SET." 

I believe the upper-level Vandersteen's have built-in powered subwoofers.

I've never been a fan of built in powered subs either,  I prefer running my mains full range and using a separate powered sub for below 50 or 60 Hz depending on the speaker.  Every powered sub speaker I've ever heard sounded muddy and colored to me.  

When you say the amp inside the speaker is never put under a reviewer's microscope, that isn't entirely correct. When they are reviewing powered speakers, they are reviewing the amp when they review the amp and speaker as a whole, how the interact together and what sound they make together. It is a review of the amp contained in the speaker when the review reports how it all works together. I think that is the key rather than isolating one part of the speaker over another, it is how it integrates as a whole, whether it was successful or not.
Mike  

A good read on the "why's", "what's" etc. relevant to the OP

http://ultrafi.com/why-everybody-needs-a-good-subwoofer/

August 3, 2008 by ultrafi in Tips, Tricks & Info 

Why Everybody Needs a Good Subwoofer……And Why a Really Good Subwoofer is so Hard to Find....

Audiophiles and music lovers are missing out on one of the most dramatic improvements they can make to their audio system: Powered Subwoofers. Most audiophiles won’t even use the word “subwoofer” in public, let alone plug one in to their precious systems. There is a kind of snobbery that exists in the world of high-end audio aimed primarily at receivers, car audio, home theater and especially subwoofers. As a matter of fact, subwoofers are responsible for many people disliking both car audio and home theater, since it is the subwoofer in both of those situations that tends to call attention to the system and cause many of the problems.

The truth of the matter is that subwoofers have fully earned their bad reputation. They usually suck. Most of them sound boomy, muddy and out of control with an obnoxious bass overhang that lingers so long as to blur most of the musical information up until the next bass note is struck. We have all had our fair share of bad subwoofer experiences, whether it’s from a nearby car thumping so loud that it appears to be bouncing up off the road, or a home theater with such overblown bass that it causes you to feel nauseous half-way through the movie. You would think that high-end audio manufacturers would be above all of that, but you would be wrong. In many cases, their subwoofers are almost as bad as the mass-market models because they too, are trying to capitalize on the home theater trend that is sweeping the land.

You see, it’s very difficult and expensive to build a good subwoofer. One reason is that a sub has to move a tremendous amount of air, which places big demands on the driver (or drivers). Moving lots of air requires a lot of power and that means an amp with a huge power supply, which can cost huge money. Finally, in trying to move all of this air, the driver (or drivers) which operate in an enclosure, create tremendous pressure inside of the box itself. The cabinet walls must be able to handle this pressure without flexing or resonating. Building such a box involves heavy damping and bracing which gets very expensive. When you consider these requirements, you quickly realize that it is virtually impossible to build a really good subwoofer (I mean good enough for a high-end music system) for under $1000. Yet most of the subwoofers out there sell for between $500 and $900. Manufacturers do this because their marketing research has shown them that that is what people want to spend on a sub, never mind the fact that what people want to spend and what it takes to get the job done right may be two different things. The result is that even most high-end manufacturers are putting out poorly constructed subwoofers that just don’t sound very good.

I don’t want to give you the impression that anyone who really wants to can build a good subwoofer so long as they are willing to throw enough money at the problem, because that really isn’t true either. There are some pretty expensive and well-constructed subwoofers out there that you would never want to plug into your music system because they would most certainly make the sound worse. Why? Because of their crossovers. A crossover is inserted into your signal path in order to remove the lowest frequencies (the deep bass) from your main speakers so that they no longer have to do all of the dirty work. The deep bass will instead be dealt with by the subwoofer. The #1 benefit of adding a high quality subwoofer to your system is not how it further extends the bass response, but how it can dramatically improve the sound of your existing power amp and main speakers from the midrange on up. That, my friends, is by far the most compelling reason to add a sub to your high-end music system. Once your main speakers are freed from the burden of making deep bass, they will sound cleaner, faster and clearer, especially in the midrange and midbass. They will also image way better because there will be far less air pressure and therefore resonance and vibration affecting their cabinet walls. And since the power required to make the deep bass is provided by the subwoofer’s built-in amplifier, your main power amp will be free from that burden and begin to sound like a much more powerful amplifier. The one big problem with all of this is that you need a crossover to roll off the deep bass in your system and achieve all of these benefits. And the crossover that comes with almost every subwoofer on the market will cause more damage to your signal than can be overcome by these benefits. That is the main reason that audiophiles refuse to consider adding subwoofers, even very expensive ones with well built cabinets.

Enter the Vandersteen 2Wq 300 watt powered subwoofer. This is the only subwoofer that is specifically designed to be inserted into the highest of high-end music systems without doing any harm to the precious signal. So how does Vandersteen do it? Simply. In fact his crossover scheme is so ingeniously simple that it’s a wonder nobody else thought of doing it the same way. I’ll spare you an in-depth description and just say that the only thing you end up inserting into your system is a couple of high quality capacitors. That’s it, nothing more! No additional wires or gadgets enter your signal path. Hell, you don’t even have to disconnect the wire between your amp and speakers to add this subwoofer. The model 2Wq sub uses the same basic crossover scheme as the $15,000 flagship Model 5As. As a matter of fact, you can even run the specially designed Model 5A crossovers (M5-HP) with the 2Wq if you want the most transparent sound imaginable.

So what about the other reason to add a subwoofer to your system: for more powerful and extended bass? I don’t care how big your main speakers are, they’re no match for a good subwoofer in the bass. A really good subwoofer can run rings around the best floorstanding speakers when it comes to bass extension, power and control because it is designed to be good at that and nothing but that, whereas main speakers have to be good at higher frequencies as well. Ideally, you want two subwoofers so that you have true stereo separation down deep into the bass. Stereo subs can also help to lessen room interaction problems by providing two discrete sources of bass information. Remember, if you can’t afford to buy two subwoofers at once, you can always add the second one later. Adding a pair of 300 watt powered subwoofers is exactly like adding a pair of 300 watt monoblock amplifiers to your system and upgrading to a pair of better main speakers at the same time. The beauty is that you don’t have to replace your main power amp or speakers to do it.

But there is a problem here as well. Everything comes at a price, and the price you pay with most subwoofers is that when you add them and their built-in amplifiers to your system, they don’t tend to blend or integrate well with the sound of your power amp and speakers. This is especially true if you own a tube amp, because the character of your amp is nothing like the character of the big solid-state amp that is built into most subwoofers. The result is that your system sounds split in half. You can hear where one part of the system leaves off (namely your amp and speakers) and where the other part takes over (the sub and its amp). This is a HUGE problem for audiophiles who aren’t willing to destroy their system’s coherence for additional power and bass extension. Fortunately, Vandersteen has the perfect solution for this problem that is, again, so simple, I wonder why nobody else thought of it first. His solution is to build a very powerful 300 watt amplifier that strictly provides the huge current needed to drive the subwoofer. You can think of this amplifier as only half of an amplifier; or just the power portion of an amplifier. The release of this power is controlled by the signal that is provided by your power amp. Vandersteen’s amplifier needs a voltage to modulate its current output, and what better place to get that voltage than from your main power amp? This way, your power amplifier is directly responsible for the sonic character of the deep bass coming from the subwoofer because it provides the necessary voltage signal. This voltage signal contains the unique and characteristic sound of your main power amplifier and insures that that character is maintained in the sound of the subwoofer itself. The beauty of it is that your amplifier is only providing a voltage reference and no actual current, so it is not taxed with the burden of “driving” the subwoofer in any way. As a matter of fact, your amplifier doesn’t even know that the sub is connected to it. The 2Wq’s potential is almost unlimited given that it will ratchet up its performance as you improve your power amp. Remember that you always want your subwoofer to sound just like your power amp. No better, no worse. NO DIFFERENT!

After having spent time with the amazing Vandersteen Model 5A loudspeakers with their 400-watt powered, metal cone subwoofers, we were reminded of the sound we had with the awesome Audio Research Reference 600 mono power amps. With the Ref 600s there was a sense of effortlessness, openness and unrestricted dynamic freedom that we have only otherwise heard with live unamplified music. Listening to those monstrously powerful amps made us realize that all other systems sound compressed by comparison. Only when we heard the new Vandersteen Model 5As with their hugely powerful built-in subwoofers, did we again have a strikingly similar sonic experience. The reason is that the Model 5As provide a total of 800 high-quality watts, to which you have to remember to add the power of the amp we were using, the ARC VT-100, at 200 watts. This means we were listening to about 1000 total watts of amplifier power – not far from the 1200 total watts provided by the Ref 600s. With the Vandersteen subwoofer crossover and amplifier, you are able to get those hundreds of subwoofer watts to blend seamlessly and even take on the character of the ARC VT-100. It’s amazing! What’s even better is that the price of the system with the Model 5As and the VT-100 is under half the cost of the Ref 600s alone! Since this discovery, we have achieved the same kind of unbelievable dynamics and seamless blending with ProAc loudspeakers and twin Vandersteen 2Wq 300 watt powered subs. So, if you want the sound of Ref 600s but cannot afford them, buy a pair of Model 5As or your favorite pair of ProAcs plus a couple of 2Wq subwoofers and mate them with a VT100 and you’ll get surprisingly close. You can cut the cost even further by running a pair of Vandersteen 2Wq 300-watt subwoofers with your existing speakers. Or mate a pair of 2Wqs with your favorite ProAc. In any case, it is the magic of SUBWOOFERS that allows this to happen. It is for all of the above reasons that there is only one subwoofer in existence capable of integrating seamlessly into a high-end music system, allowing you to reap all of the benefits of having a subwoofer, with none of the drawbacks. And the Vandersteen 2Wq is the one. And just in case you think I am a biased source, our correspondent Blaine Peck (who, for all you know is also a biased source) recently wrote the following, with no discussion between us about the topic prior to his sending us his comments. Whether reproducing the plucked string of an acoustic bass or the sound of an analog synthesizer, the Vandersteen 2Wq subwoofer is a seamless extension of any system. Nothing else need be added! With its internal 300-watt power amplifier, it is the perfect compliment to any sound system. Designed to take on the characteristics of your main stereo amplifier, the amp in the 2Wq will not sound foreign in your system. Also, through an extension of the Vandersteen design philosophy, a unique gradually sloping crossover system is implemented so you simply do not know where your main speakers stop and the 2Wq begins.

Now that your main speaker/amplifier combination need not concern themselves with those power demanding low frequencies, they are freed up to work in a more comfortable range. Yes, now what is coming from your main speakers will sound better than ever.

The 2Wq is not just another subwoofer. It consists of three 8″ floor-facing drivers, each with a massive motor. So why not a more typical single 12″ or 15″ design? Well frankly, the mass of a larger driver will not allow it to respond as quickly as the Vandersteen 8″ drivers to today’s demanding recordings. The 2Wq’s 8″ drivers are designed to handle the content but be “fleet of foot” at the same time. Concerned about where to put them? You need not worry. With the control of both its respective level and the “q” (how loose or tight the low end is) you have the flexibility to place them in a location that fits your living environment and not sacrifice performance. The simple beauty of this product will soon become an addition to your room.

So whether on orchestral music, hard rock or something in between, the Vandersteen 2Wq will exceed your expectations.

It makes some sense to provide a powered woofer for certain kinds of systems.  If the midrange/tweeter assembly is quite efficient, but, the designer cannot find a suitable woofer of equivalent or higher efficiency, a conventional design would mean having to pad down the tweeter/midrange to match with the woofer and that would waste the higher efficiency of the other parts of the system.  With an active crossover and powered woofer, the system owner can use a much lower powered amp for the top end.  

This is a VERY important feature of speakers, like those made by Avantgarde, because very low powered amps, when used with suitably efficient and easy to drive speakers, sound great.  I have not heard ANY solid state or tube amp that sounds better (for my taste), than good low-wattage pushpull or single-ended tube amps.  

I do not like the idea of subs built in to full range speakers and will stay away from them. I do not like the idea of limited subwoofer placement. 

This is a good deal if it suits application.  
https://www.audiogon.com/listings/solid-state-anthony-gallo-acoustics-ref-3sa-sub-amp-2016-04-18-amp...
http://www.roundsound.com/reference-sa-amp.html

I heard some entry level 600 series B&W speakers. The 605, with a sub amp and speakers, was able to produce a greater sound level with a smaller main amp. Also, crossover losses are less. I have some old B&W DM 1400's This speaker was also made with built in amps and sold as the Active 1. It played louder and cleaner than with a Phase Linear 400 on the 14's.

I have main speakers that are 3 db down at 20 hz.  I added a couple of powered subs that go to 16 hz for movie watching, but listen without subs to  just music.

A lot of so called full range speakers do not deal well with bass below ~35 hz, which is fine as long as you don't know it is missing.

My 3rd system has Vandersteen 4s that have separate bass amplification.  I believe Richead went to on board amps in subsequent models to ensure quality of bass (people over driving unsuitable amps for the bass)

While onboard powered subs in a speaker can cause problems in optimal speaker location, their voicing, being that the manufacturer has control of that aspect, trumps the separate sub or subs in some senses. One such instance is the ability to integrate the sub into the overall frequency response pattern, a benefit which answers to some degree the main drawback of adding subs to a system. Cabinet resonance must be dealt with and getting the best ball response and imaging from the same location requires careful placement. In my experience the placement of this type of speaker is even more critical than normal. This was brought to my attention by Jim Smith and I must say he was on the money here. 
Nodes are another factor where having the subs in the same location as the mids/tweeters can cause issues. But to be sure dialing in the correct placement of separate subs can be frustrating as well, not to mention speaker cable placement and ability to have the subs in a user friendly location. 
All in all, I worked it out to my satisfaction and wouldn't change anything. The balance in the spectrum built in by the manufacturer overcomes the downsides, at least in my experience.

I've yet to find an amplifier that plays bass as well as the amps I have. So I tend to prefer speakers that have deep bass response. The speakers I have at home go to 20 Hz and are easy to drive- 98 db, and my amps have full power to 2 Hz so I can shake my home easily enough!

But my speakers are expensive and I've no worries if my stereo takes up space in my living room- I'm involved in the audio industry after all so it had better! But that is not the case for all people of course so a power sub can be really handy for keeping the sprawl under control and also getting some decent low end extension.

I do find though that if a sub is used, one needs to exercise a lot of care setting it up, otherwise blend issues will plague the system. It is precisely that which is why I prefer full range speakers, so I don't have to deal with that.

A general rule of thumb is all about a dedicated subwoofer amp may perform better on low frequencies than uber-awesome full range amp.