Speaker sensitivity vs SQ

A low sensitivity speaker can be very bit as dynamic as a high sensitivity speaker. It is just a matter of power.
Has anybody ever wondered by no two loudspeakers sound alike? Assuming there is only one accurate sound, that would mean that everyone except for maybe one company has it wrong. More probably nobody has it right.  There are so many varied considerations in speaker design that it is difficult to be versed in all of them. So, we are left to art and our own devises, hearing. We all have our theories and preferences which is what makes this fun. 
I personally don't care at all about efficiency. I separate sub bass from everything else because the considerations are so vastly different. I care about directivity, uniform radiation and radiation type (line vs point source). Because of the directivity mandate I prefer ESL line sources and horns (point source), one moderately inefficient 89-90dB/watt/meter and one very efficient 100+dB/watt/meter. Both are a much better impedance match to air thus the transfer of sound from the driver to air has inherently less distortion. Dynamic drivers sound rounded off to me as if the tips of the transients are missing (no idea if this is correct or not). Many like that "smoothness" and certainly prefer the size. For many it has been the only type they have been exposed to. 
ESL line source speakers are a bit tricky as they do not conform to the usual rules. Even though they are less efficient, because they project power into the room better than a point source an ESL playing at 90dB at 1 meter will be substantially louder and more dynamic back in the room. Than a dynamic speaker playing at 90dB at 1 meter.
Because small ESLs can be rather timid I am of the opinion that if you are going to do ESLs get them 8 or 9 feet tall (depends on your ceiling). It is a major difference in performance, so much so that a pair of subwoofers will never be able to keep up with them, takes at least 4.
As for subwoofers, we have been blessed with modern drivers, amps and crossovers. With enough power you can make a subwoofer do almost anything you want within it's volume constraints. The real problem is the room. If you have to you use multiple units and keep the crossover below 80 Hz. With some speakers like planar magnetics and ESLs there are advantages to crossing higher which means placing the subs in a symmetrical array otherwise you can put them anywhere as long as they are up against a wall. If you put a sub in a corner it is going to be louder than the others.
Can you make an efficient subwoofer? Sure, I made one, or rather we made two. We put 30" Hartley woofers in 25 cubic foot enclosures and drove each one with 20 class A watts. Ridiculous would be an understatement but it worked. You can not use most modern subwoofer drivers in large enclosures. What happens is the voice coil bottoms out making a very disturbing sound. You could use multiple drivers so that each one sees a smaller volume but you still have a very big speaker. There is this thing now with dipole subwoofers. If you have been mislead to think these work just measure their performance. The data will make you cry. There is no circumstance under which a dipole subwoofer will perform reasonably flat from 18 to 100 Hz. I've never measured the efficiency of one but given the degree of cancelation going on I can't imagine that it will be very good. 
IMHO  full range drivers by themselves do not cut it. You can get a very nice midrange and if you lock your head dead on with the driver some treble and in a transmission line enclosure even a little bass. You can do better with Parts Express, Madisound and a little smarts. Horns are great as they are directional and do not bounce sound all over the place. I love the visual statement some of the systems make. There is nothing cooler than those multicolored and wooden horns. Reminds me of "His Masters Voice" and boy do they go loud. If your thing is pee-watt amplifiers these are definitely your speakers. Good horns will not honk at you like PA speakers and like ESLs the micro-detail and transients are excellent as both are much more efficient at transferring sound to open air. Listen to a drum solo and those snare drum snaps. 

Phusis and Duke, I beg to agree and disagree. I have always said the more power the better. This is a relative statement, relative to efficiency and other factors such as the output capability of the speaker. A speaker can only do so loud.
I have been listening to as many horn loaded loudspeakers as I can lately and in general they are very impressive or can be very impressive. It is certainly easier to get dynamic sound because they are so efficient and they go very loud. Now an ESLs volume capability is based on its Xmax which is very small in comparison to dynamic speakers. If you try to run it full range with bass laden material it will run out of Xmax pretty quickly and start clipping or rapping the stators. However if you do two things the situation turns upside down. These are, send everything under 100 Hz to a subwoofer array and design the speaker so that it is a full range line source. What you get is every bit as dynamic as a horn system and I think because there are no crossovers otherwise, an effortless naturalness that makes the speakers disappear. Line sources project power better than point sources. Because ESL almost match the impedance of air their transfer of power is very efficient even if their electrical efficiency is not. So, you have a very dynamic, low efficiency speaker system that goes very loud and has no crossovers above 100 Hz.
Phusis, lets say you have two speakers that both clip at 120 dB at 1 meter. One has an efficiency of 103 dB/1watt/meter and the other 84dB/1watt/1 meter. With a paltry 2 watts the first is blasting at 106 dB and the second only at 87db. To get the second to 106 dB you need 160 watts and this is at 1 meter. At a reasonable listening distance you are easily under 100db, probably down towards 95 dB. We are not even talking about peaks here. The point is that low efficiency speakers require a lot more power to hit dynamic peaks, hundreds of watts. Duke, your friend has more work to do. There are so many factors involved that I doubt you can make a blanket statement that high efficiency speakers are all more dynamic than low efficiency speakers of various types given appropriate power. It is certainly easier to make high efficiency speakers sound dynamic and I would rather have a good sounding high efficiency loud speaker than an equally good sounding low efficiency speaker. The more power on a relative basis the better. Fortunately for me ESLs are more efficient than ever 89-90dB/1watt/meter and there are plenty of amps now that can drive them without farting or blowing up. I should be able to hit 100 dB without leaving Class A operation. 

Sorry Duke, my bad. My point is that dynamics are a matter of volume. A speaker that can hit 110dB without compression is going to be more dynamic than a speaker that can only get to 100 dB even if it is less. efficient. Just a matter of power. Horns are very dynamic because they go very loud. They do it with less power because they tend to be very efficient. As far as sound quality goes, it's a toss up.
Another issue is trying to run 15" woofers up to 700Hz then crossing to a horn. Two very dissimilar drivers crossed right in the meat of the midrange. You really have to push that crossover lower and in order to do it you are stuck adding a tweeter. I'm  all in for limiting crossovers but horns have their limitations in this regard. This is what makes ESLs so special. You can easily get them to go from 100 Hz to 25kHz. Obviously you can make them go lower but IMHO you are much better off going to subwoofer especially if you can use a digital crossover. 
Audio2design, I know for  fact  you do not have to have active loud speakers to take advantage of digital signal processing. Perhaps it will be the way the market goes but I am not so sure and I am an unabashed huge fan of digital signal processing just check out my system page.
Alexberger, in order to have inertia you have to have mass. Thermal distortion (whatever that is ) certainly has no mass. You are more than welcome to your SET amps. They are most definitely not my cup of tea.
I have heard several speakers in an anechoic chamber and they sounded just fine. You can test your own by just moving them outside. Tuning a speaker in an anechoic environment is a fine way of achieving a flat curve or rather the curve you want. In order to tell what your speaker is really doing it is the only way. What happens in a room depends on the speaker's dispersion characteristics and the room itself. A speaker that is more directional will sound very similar in room as in an anechoic chamber. Controlled dispersion is the best room treatment you can get followed by digital room control (speaker control) and a few foam tiles. 

Alex, who said you need 100 watts to drive a voice coil at normal volume? First of all music is dynamic and made up of a palette of frequencies. You don't listen to a sine wave. Even with less efficient speakers RMS wattage is going to be pretty low at normal listening levels maybe 10 watts at most while still hitting peaks of 100 watts. A woofer that is well ventilated is it's own fan. That is the way it is designed. The drivers I use can take 600 watts continuously indefinitely. I doubt they have ever seen 100 watts RMS 

1+ ctsooner. The most expensive speakers I have even seen are not efficient at all. The Klipsch Horn is actually a good value in today's market.
@audiokinesis, That makes perfect sense Duke. The radiation of the woofer will narrow down to that of the horn buy 700 Hz. It should work as long as the driver is not colored. The speakers that I have heard that run a 15" woofer that high have been colored. Again the K horn is a good example and I think they moved the lower crossover to 350 Hz from 500 Hz. If you manage to avoid another crossover point that would be great.
@atmasphere, I do not disagree. Thermal compression exists but good drivers take this into account, ventilation, ferrofluid, etc. So, to what degree does this effect the sound we hear at home, at the volumes we listen at. The only drivers I have are subwoofers and with the system going as loud as I'll ever play it they are just loafing along. I would think ESLs would be more limited by mechanical compression than anything else. I'm not sure but I do not think they heat up at all. The transformers certainly stay cold. Transformer saturation might be an issue.

@audio2design, I doubt anyone is going to jamb a tube or class A amp into a speaker enclosure. You can add DSP control to any system. Check out DEQX and Trinnov. The problems with active loudspeakers are, audiophiles tend to shy away from all in one stuff. Right now this market is with equipment like Sonos. Great for TV and background music, not to hot for overall sound quality. The companies that make this stuff and have the horsepower to do it are not interested in audiophiles. Not enough of us. They are marketing to the masses who do not like big speakers, do not want to spend a gazillion dollars on equipment and are not critical listeners.  I also doubt they would ever go near an 8 foot ESL. Amar Bose was a brilliant guy but lets face it, the 901 was not a great speaker. He sold a bunch of them jumpstarting a huge company with lots of horsepower. Look what they make now. How is Meridian doing? You certainly do not see a lot of their systems in the US. Audiophiles have a tendency to be digital and processor phobic. The last thing a died in the wool Vinyl listener wants is a computer in his speaker.

@phusis, yes, my set up is fully active but you could never use SET amps in my system. I really require big amps. I was talking in the context of very efficient speakers. SET amps are very romantic and if that is what you like then by all means but a class A SS amp particularly a Pass or Curl design is going to be more accurate, have more control over the woofer and still have a little of the romance. I would also rather buy records than $1000 tubes.

They are also a lot nicer looking!!

Phusis, I would bet there are a host of SS amps that would handily out perform any SET amp. Any small class A Pass amp for sure. JC1's definitely and most probably all of Atma-Sphere's amps. Like trying to race a 911 in a VW Bug.

My opinion of speakers is very jaded now. I've been living with large ESLs for so long that most other speakers sound.....small, even if they go loud.
I find horn loaded designs very interesting but I have yet to hear one that I would consider. I have not heard Ralph's speakers, I'm sure they are excellent. I have heard a few at shows that were not impressive, but what ever is at a show. Some of the older Horn loaded speakers like Altecs and JBLs that I had a lot of experience with when I was young were all very colored. Back then K horns were the best but they did not image well and though not as colored as the other horn speakers you still knew you were listening to horns. Frankly, I liked the Cornwall better than the K horn.
I owned Heresy's for 10 years and for a teenage rocker with a Dynakit Stereo 70 they were great until I got Large Advents and a Phase Linear 700. The Advents were more natural and imaged very nicely. 
The Heresy's were the last and only horn speakers I ever owned. The problem is there are painfully few situations where you can hear them in a controlled situation. Some of them are very large, very expensive and few dealers are going to keep them on the floor. 
Those of us here have gone down our own path and become entrenched in one technology or another and always seem to think ours is best.
There is no single best. There are thousands of bests. Since we can't hear all of these systems it is fun to talk about them and hear about what other people are doing. In many instance you might learn something that you can apply to your own system.

Atma-Sphere that sound great but is way above my pay grade. But, I think you just explained why I like Pass and Curl amps so much?
Phusis, those are quite some speakers you have. What are you using for subs? They must be unusual to keep up with your main speakers.

They are very interesting speakers and IMHO very nice looking. It is a world away from what I deal with.

My approach to subwoofers is totally different to Phusis. His are 7 dB more sensitive but much larger 20 cf to my 1.9 cf ! I have to use four of them to create a line array and each one has two 12" drivers in it. They require power and a computer to control them but they will go flat down to 18 Hz. The enclosures being small and cylindrical are very stiff and the cabinet resonance is way above the operating range and very well damped I can't even see it running them full range. They break up at 2000 Hz. Actually, I should say "it" not "they" as there is only the mule built. The idea was to minimize enclosure vibration and resonance using woodworker friendly materials. The woofers are in phase and located at opposite ends of a 28" cylinder. Their reactive forces cancel out keeping the affair from shaking. The walls vary between 1 7/16 to almost 2". I have not weighed it yet but it is darn heavy. Definitely a two person lift. Now I have to build the other three and finish them all in polyester satin black. They should have no problem keeping up with the Sound Labs.
Psyched to say the least. ESLs are so simple relative to other designs. There really is not much to them. Working around their limitations is pretty easy. You just need the right amp and you have to take the low bass away from them. They will do it but it so compromises them.