Is powerfull Amps only for low sensitivity speakes?

All I can add is personal recent experience with a 500 watt Magtech amp compared to a my 70 watt cj tube amp. I was worried the 500 watts would be too much for my 90dB efficient Spendor SP100 speakers, maybe even blow them out. What I should have worried about was having to return the Magtech!  By comparison, The cj sounded so muddy and hazy, especially in the lows. For many years, I thought the Spendor’s were slow and had difficulty with a lot of more upbeat music. Now I know it was my under powered tube amp. The extra watts gave a much more overall relaxed, detailed, rhythmically assured presentation. I guess there is some midrange glow that the cj adds, but now it is very clear that this  effect is distortion. 

I’ve heard it said many times that you cannot have too much power, but under-powering speakers and causing the high transients to clip the signal can do damage to any speaker; therefore, it is good to have headroom for the speakers in which to have room to function and breath properly. Using a water analogy . . . the fire department wouldn’t use a garden hose to put out a large fire. Even if there was plenty of water pressure, the hose could not handle it and would probably burst, and even if the hose was strong enough to handle that water pressure (power), it would not have the diameter to produce the VOLUME of water necessary to do the job.  Having all that water pressure . . . the firemen are always prepared in any given situation to do the job; however, if the water source does not have the pressure -- no matter the associated equipment and controls, the firemen can open up their hoses at full throttle . . . and they will only get so much volume coming out, and the building will burn.  Simply put, the right tools/sources to do the right job will produce the desired results and without disappointment -- if used with the proper knowledge, skills, and wisdom.

In the case of the power amp(s) being expensive, there are some class D amplifiers, that have capabilities of taking on various efficient to inefficient speakers like from 2 ohms up to 8 ohms or more with no sweat. In fact, some do so with great sound and not a high end price . . . i.e. a Crown XLS 1502 is rated at: watts/side @ 8 ohms: 330W; watts/side @4 ohms: 525W; and at watt/side @ 2 ohms: 775W. So you see, there is plenty of headroom, and this amp has level limiters so that you can set the highest limit of the amplifier’s volume to match the volume control of your preamp or input device so that you will not blow out your speakers or your ears. They have one model that has less power . . . and two models with higher power, I believe, in the XLS model.

In any case, it is always nice to have options and at reasonable prices. I have relatively inefficient speakers running at approximately 8 ohms and rated for up to 200 watts of power, and this amplifier, which can also be bridged into even higher power, has absolutely no problem running these speakers. In fact, I use a DBX to control my sound system to tune it to the room and have control over the way the speakers sound, and have thought about turning my speakers into active speakers with the DBX acting as an electronic crossover and using Crown amps for each frequency band. Right now I have the Crown operating the main speakers and my old monoblocks B&Ks (250 watts each) running the subwoofers, and the blend is seemless as the levels between the two. can easily be controlled. It is always nice to have options and versatility . . . and the Crowns do give you exactly those options.

My experience is that inefficient speakers just never quite sound right to me, regardless of how much power the amp supposedly has. They always sound somewhat sluggish, effortful, leached-out, and, my horror-words in the audiophilic realm: "neutral" (means: bland) and "transparent" (means: thin). 87dB is the lower limit for me, 89dB is better, over 90dB is better. Then I'm free to use tube amps, EL34s, single-ended Alephs, sweet Creeks, integrateds, whatever I want. To low-sensitivity speakers I say "bah, humbug." I've yet to meet a high-wattage amp that honestly sounds really good, and I used to work in a high-end audio salon so I've heard amps that cost up to $25,000/pair. In my mind they just get you back to where you would have been in the first place if you'd gone with speakers of decent efficiency. Just my personal opinion according to the ears attached to my own head, YMMV.

It should be no surprise that 15" drivers, regardless of their so called efficiency (or sensitivity), need higher voltages (or power) to operate at their best. You still need to move that large amount of air and that takes juice. In my opinion (hopefully a fact), the reason why a tube amplifier may sound a bit more "muddy" in the low octaves when compared to an SS amplifier is not (necessarily) due to the power rating, its mostly because of the typically much lower damping factor of the tube design. That usually translates to a "looser" bass. You'd have the same result even when you compare the amps - tube vs. SS, with identical power rating. The OTLs are exceptions because they tend to have an un-tube like very low output impedance thus a higher than average damping factor.