Is powerfull Amps only for low sensitivity speakes?

Power is relative.  My wife has a Mini Audio system. Cheap,Cheap!  For what it is a $300.00 complete 10 CD changer, Radio, and 6" small bookshelf speakers.  It is rated at 100W RMS/ channel. the unit will get very loud, however it has no balls.  I have a 50W RMS / channel Crown PL-1 (22,000mf reserve capacitance) and in comparison at the same volume level the difference in the dynamics is astounding.  Playing on speakers of 89db /w 1W efficiency and driving about 20W the punch and low end off of the Crown amp is like night and day compared to a double the power rated amp in the cheap system.  It is like comparing a 50W crown to a 50W Mark Levenson amp.  Even though both are rated the same wattage the Levenson power supply would have I am guessing triple reserve in the power supply.  Not to mention the quality of the components from front to end.  So we can talk about watts but that also does not tell how it sounds. Generally you get what you pay for but I always recommend listening side by side on the same equipment.  The lesser wattage amp might sound better depending on what you like.  That is what matters - what you like. 

Someone wrote:
"These statements are not accurate. If the amp makes too much power and the speaker has no need for it, the amp will be operating in a lower power region. With most higher power amps, this means it will have increased distortion. You can see this in their specs. The increased distortion is audible as brightness in most cases and will result in less detail as the distortion will mask lower level signals."

Not True! I wont even start here. I will ask you to think about percentages and hwo the FTC requires amps to be measured.

Power is so so overrated. I have Magnepan 20.7, 86db and I’m currently driving them with a 90wpc tube amp without a problem and to be totally honest they have never sounded better. I use to drive them with ARC DS450M mono-blocks that put out a whopping 450wpc and 600wpc in a 4 ohm load. And to be frank they sounded like crap. I have never listened to music as long as I do with my current setup. Buy a well built amp that has bandwidth and high current and pay less attention with wattage. Just my humble opinion. Let your ears be your guide and not hyperbole. 

May I ask your comments also for the difference between single ended triode amps and push pull tube amps in terms of 2nd and 3rd harmonics capabilities.

Its not as simple as that might sound! SETs tend to make more of the 2nd harmonic than any other, with each successive harmonic at a much lower level than the one preceding it. The 2nd harmonic is where the 'richness' of single-ended operation comes from.

If an amplifier is push-pull, the harmonic structure varies depending on how the push-pull is accomplished. For example, if a single-ended input is employed, the 2nd will be canceled in the load, but there will be some present on account of the input, and you will also see a 3rd and a fairly prominent 5th. Many people that prefer SETs often point to the distortion of traditional push-pull tube amps as most are built this way, as being why SETs sound better.

But that is not the full story.

If the push-pull amp is entirely differential and balanced from input to output, the 3rd harmonic will be the primary distortion component but at levels about the same as the 3rd is seen in an SET. So this is considerably lower distortion and such an amp will tend to sound less romantic and more neutral, but will retain much of the musical character of an SET.

So topology plays a pretty big role and you really have to know what that is about. The 2nd, 3rd and 4th (the lower ordered harmonics) are musical to the human ear. The 5th and above tend to contribute to brightness and harshness, for example we've known since the 1930s that the 7th contributes to a metallic quality. Our ears are particularly sensitive to the higher orders and use them to sense sound pressure. All forms of distortion are perceived as some form of tonality.

"These statements are not accurate. If the amp makes too much power and the speaker has no need for it, the amp will be operating in a lower power region. With most higher power amps, this means it will have increased distortion. You can see this in their specs. The increased distortion is audible as brightness in most cases and will result in less detail as the distortion will mask lower level signals."

Not True! I wont even start here. I will ask you to think about percentages and hwo the FTC requires amps to be measured.

Actually the statement was quite true and simply looking at the distortion specs of the amp in question will bear this out. The FTC really doesn't have much to say about it. They are more concerned with distortion at full output.