how can low watt tube amps drive speakers with higher power requirements

I am new to hifi and I am super confused about something. Most audio blogs out there ask newbies to stick to amps that output power within the recommended range of the speaker manufacturers. However, on forums, blogs and even some magazine articles, I find pros reviewing tube amps with much lower output power (even in some cases 10-30W below the speaker specs) and find no problems. How can these low power tube amp drive these speakers? For example, the LS 50 metas spec sheet says "Recommended amp power: 40W - 100W) but I have seen posts here and on other forums where people will hook these up to tube amps producing as low as 12W of power at 8 ohms. Am I missing something?
You are indeed. And you are not alone. Whole lot of guys are missing it. Even ones who should know better. Amplifier watts are just about the least relevant spec in all of audio. 

What really does matter is speaker sensitivity. Because it requires a huge increase in power to play just a little bit louder. 3dB is a small increase, but it requires TWICE the power! Think about that. 10dB, TEN TIMES the power! 

What this means is an amplifier TWICE as powerful will only play a measly 3dB louder. No matter what speakers. That is a fact. 

So if you buy 88dB speakers, they will need 100 watts to play as loud as a 98dB speaker will with only 10 watts. 

This is the reality. Speaker manufacturers find it a whole lot easier to sell tiny little speakers, because they know women control the speaker market. Tiny little speakers are horribly inefficient so they hide this from guys they know are too whipped to do what's right by their audio anyway.

The fact of the matter is no speaker has any power requirements whatsoever. What they mean by this, their convoluted logic, is they know their speaker is so terribly inefficient nobody is gonna be happy with less than 50 watts to bring it up to a reasonable level. So they say 50. Then they also know their speaker is gonna burn out if played loud with a lot of power. So they say 50 to 200 watts. Or something like that. Point being it is all smoke and mirrors. You can safely ignore the whole thing. It is pure BS.

All you need to know is if you buy speakers with sensitivity much below 92dB you are going to start to have a hard time finding an amp you can afford they will sound good with- and the further below 92 you go the harder - and more expensive- this will be. Stay up around 95dB and above, no problem. All kinds of amps to choose from. Some of the very best amps at any price only put out around 10 to 50 watts. Which is all you need, provided only you are smart and avoid anything below about 92. All you need to know.
thanks for your response! this makes sense -  however, some literature out there also says that "low power" damages the speakers -  is there any truth to this?
Low power from the power amp or the Voltage/amperage from the power being supplied to your equipment? Lower volume should prolong the life of most any driver. Low voltage on the other hand is a death sentence for most modern equipment without some type of 120 vac maintainer... Surges are bad, brownouts are worse...

If you push an amp into clipping trying to eek out more volume from an undersized amp then you can damage a speaker. A monster sized amp means it’s still just barely ticking over at volume likely to make your ears bleed. You’re more likely to damage your hearing than your speakers with too much amp.

There are SPL meter apps you can download for your phone. Check how loud your typical listening level is. Add 10dB for dynamic headroom and calculate how much amp you need. There are calculators online to help you compensate for distance and room reinforcement. Be careful believing published specs. Sometimes they are optimistic. Also there can be adjustment for impedance and whether the spec is given as 1W or 2.83V (1W @ 8 ohm).
"Back in the day" we bought stereo systems and the first thing asked was, " How many watts?" Every manufacturer played the numbers game to perfection because we had become "spec sheet scientists". {RMS @ 8 Ohms 20 - 20000...THD 0.000001 S/N ratio. Wow and Flutter. You know the drill} Whatever....we used to kick car tires too. Tonight my kit blasts out a pathetic 20 watts per channel. It runs fairly high distortion levels relative to most. I only have two channels. Two speakers. No sub. One turntable. One CD player. Simple, endlessly astounding, intimate, a sound stage to die for, with totally lifelike audio reproduction, (IMO) I live in my happy place. Why did I take such a long route to achieve such an elusive yet rewarding destination? I think because I wasn't "listening" , I was measuring, counting, and competing. Just sayin' (Wish someone had told me that a long time ago)
No you are not missing something.
Any amp will work. It’s just a matter of how well and the results will vary widely.

The main thing always is to avoid clipping. That is public enemy #1.
Tube amps typically soft clip as do some others.  You seemingly get away with fewer watts when soft clipping but the result is distortion nonetheless.

My 91db speakers are driven just fine by my 20W tube amp.  (I will say, though, that with a similarly-priced SS amp at 40W, they sounded like sh*t.). I doubt that there's a general rule that you could follow that would trump the judgment:  "What does it sound like?  Do I like what it sounds like?"
If you buy a lot of equipment over the years you will find out that consumer grade receivers at 100 wpc have trouble keeping up with 25 wpc tube amps. You'll also find some 60 watt amps that kill 100 wpc receivers. The difference is power supplies. Good power supplies provide great experiences. The best way to stay on course is keep away from consumer electronics and buy quality audio from know good specialty manufacturers. 
The other things that make a difference in regards to how much power is needed is how loud will you go and how far away from the speakers will you be. 
So much of it depends upon your room size and how loud you want to listen.  Obviously the larger the room, the more power you will require to fill that room.  The same for high volume listening.  If you are using a smallish room and never go louder than 80dB, then you really don't need too many watts.  BTW, 80dB while not considered loud is too loud to have a normal conversation.  The manufacturer's suggestion is a generalization to cover a wide range of situations that may or may not apply to your particular case.
The answer is pretty much covered by the other responses here... the loudspeaker rating is how much power it will tolerate, the amplifier rating is how much power it is able to deliver. So you need an amplifier that can deliver the sound levels you want plus a bit of headroom to avoid excessive distortion / clipping.
One advantage of using a lower powered amplifier is that you don't need to attenuate the signal as much... potentiometers tend to perform progressively worse the more the volume is turned down, there's more information on that here if anyone's interested.
4 Watts drives my 92db Ref 3A de Capos just fine.  Not concert level but loud enough in a 12 x 20 room with openings on the side
KEF doesn't know what kind of amp or quality of amp you're going to use.  The figures quoted are no doubt within a "safe" margin, and were probably determined as much by lawyers as by technical people.  Experiments are good, but always at your own risk.
An inadequate amplifier is easily driven into clipping and the clipped output of an amplifier includes excessive high frequency distortion energy.  It is the clipped output that is the tweeter killer.  For that reason low powered amps lacking soft clipping are more dangerous to speakers than are higher  power amps just idling along.
Aside from his stating the importance of speaker sensitivity Robert Harley states " Another electrical factor to consider is the loudspeaker's load impedance. ....The lower the loudspeaker's impedance, the more demand is placed on the power amp. If you choose low impedance speakers, be certain the power amp can drive them adequately."
I agree with the Miller on the choice Of loudspeakers since then you have many, many amplifier options. . If you start with a low powered amplifier then you are going to be limited in your loudspeaker choices. Generally speaking, lower efficiency speakers may have better bass and higher efficiency speakers are more difficult to design for good bass impact. Another thing to be aware of is the nominal impedance of a loudspeaker. Low impedances are going to require a lot more power, as a general rule of thumb. The loudspeakers I use are 99db efficient at eight ohms nominal. I have used these loudspeakers with as little as eight watts  per channel to play very loudly.
The low power amp with say 50 watts cannot harm the speaker able to handle 500 watts, unless maybe if you drive the 50 watt amp so hard that it is clipping a lot. Then if clipping is bad enough the output looks almost like a square wave. This is kind of like, imagine you are doing bench presses. Only instead of the sine wave full lift you do a square wave and try and hold it at half way. How long till your arms are shaking? That's pretty much what happens with speakers. Power pushes the cone out part way, holds it there, distortion increases, a lot of it is very high frequency, and he combination of square waves and extra high frequencies burns out your tweeter. This is why almost always the tweeter goes in these situations. 

Now if you really do use too much power, even really clean power, that is another problem. Because dynamic drivers are a voice coil inside a magnet gap, with a spider and a surround designed to hold the coil so it moves like a piston perfectly straight and on-axis. Which works fine within a certain range. Too much power and the excursion is too great, the spider or surround can flex the wrong way, the coil goes out of alignment, and it doesn't take much the gap is real small, voice coil gets bent and you start hearing a scraping sound, or maybe knock or rattle, depending on what happened. 

Your third failure mode is heat. Power heats the voice coils, and if they get too hot the insulation can burn, leading to a short. Or the coil can deform, leading to scraping, rattling, etc. 

These are the real reasons for giving power ratings. But it's like megapixels with cameras, the last thing that matters is the first thing they feed you. Because too much trouble to explain so you understand what is really going on. 

So being "smart" would mean avoiding anything made by Vandersteen?

If so, that's a real bummer as I was hoping to get a pair of these someday.
Human hearing is logarithmic. Look up dB is a log scale for the sound pressure wave (SPL). To make something sound twice as loud, you need 10x the power. Ask any guitar player and they will tell you tube watts are louder than solid state. A 30 watt Vox can easily hang with a 50 watt Marshall. Bottom line is don’t worry about watts g go or a home system. 
I build my own custom tube amps for sale and for my own use. My favorite  design is a pair of NOS 300B tubes operated in single-ended, ultra-linear mode. About 15 watts RMS output. It drives even the most power-hungry speakers st acceptable volumes for listening. It won’t; of course, blow the doors off your listening room, but; really, who needs that kind of output unless your half-deaf or a heavy metal freak?
I agree 100% with millercarbon. An audio system is something that takes energy and turns it into something useful. Think of it like an internal combustion engine, which is basically an air pump. The more air it can take in, the more air it can put out. That equates to horse power. Restrict the input and you restrict the output. Give it plenty of input with a supercharger but restrict the out put with a crappy exhaust system, and you are just defeating the whole system. That's what choosing low efficiency, hard to drive speakers does to an audio system. Choosing low efficiency speakers is just stacking the deck against better sound in an audio system. Sure, a tube amp can play with low watts and low efficiency speakers, as long as you are content with low volume. The first time you go to crank up the volume, you'll discover the error of your ways in the name of distortion. Great sound is all about speed and reserve power. A system that is fighting it's speakers has to give up speed, and that equates directly to quality of reproduction, because a reserve of power to keep up with the rapidly changing dynamics of music is what makes an audio system truly shine. Even with high wattage systems, low efficiency speakers make no sense. Your speakers have the final say in how your system sounds. Why allow them to be a road block? As miller aptly stated the true and most important spec of a good speaker is it's sensitivity, and, the higher, the better. 
It is all based on the quality of the first watt with a lot of amplifiers and speakers and the ability of the amplifier to drive the speaker. The other main factors are room size and listening level wanted. Small amplifiers can be good for a lot of music and speakers but only at low listening levels turn up the volume much at all and you will tax the small amp and you weigh the choice between volume and quality of sound with small amps. The key is to find a good all around amplifier that allows you to play whatever you want on any speaker at any volume, they do exist, but are hard to find.
A low powered tube amp is unlikely to damage a tweeter if driven to distortion, it simply sounds bad.
My favorite  design is a pair of NOS 300B tubes operated in single-ended, ultra-linear mode. About 15 watts RMS output.
Funny!!! 300bs of course can't be operated in ultralinear, on account of lacking a screen grid :)
I read that certain speakers require amplifiers with high "current" and not necessarily high wattage.....and this usually requires a tube amplifier. I believe this has something to do with 20w tube amplifiers driving speakers better than a 150w SS amplifier.
Indeed, wattage is just part of the equation.. Average watts per channel RMS (root mean square) is an average which means there will be peaks and valleys of  power output way beyond and way below the RMS value.  Amplifier manufacturers that do not state power in watts RMS per channel is quite meaningless or hard to understand what they mean.  And the quality of the first watt is super important.  For instance, Klipsch Heritage speaker line with efficiency up in the 98 and above can easily be powered by a 2.3 watt/ch  SET amplifier from Decware (see Steve Guttenburg's YouTube channel on this very topic) if you listen at a relatively reasonable level (vs a night club !).  There is an obsession on the more watts, the better. That is not necessarily true.  PLUS, some speakers have impedances that are all over the map and can approach zero ohms (e.g. almost a short circuit)  and that requires a very well designed amplifiers (e.g. David Berning's)  more than just powerful amplifiers.  
No you are not missing anything. You can get to a fairly high static output with a low powered amp on a less efficient speaker .....playing a sine wave. Music is not static. It is very dynamic. Peaks can require 10 times the output or more. Thus at volume the amp is clipping all the time and the sound is pathetic. If you only listen at very low levels it will work. I love string quartets but I could never spend my whole life listening only to them. Replace the amp with a very powerful one, 200+ watts/ch and music flows out of the speaker in an effortless and dynamic fashion. For low powered amps you have to stick with very efficient speakers but IMHO anything less than 30 watts/ch is a waste of time. Unless you only listen to string quartets. You can get SET amps this powerful if you feel the need. They are also stupid money. For less you can get Parasound JC1+'s which is like comparing an old Jenny to an F22 Raptor.