Is powerfull Amps only for low sensitivity speakes?

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.

The answer is No for low sensitivity and don't be confuse with the motion that tube amps work only with 87 or 92 db range.  What you need is high efficiency speakers that will work properly in every amp category.

Some speakers require amplifiers with a higher damping factor (DF) and some speakers sound their best with lower DF amplifiers. The desirable level of DF is determined by the speaker design and the intentions of the speaker builder.

You can increase DF and inversely lower amplifier output impedance by utilizing more NFB. Some amplifiers tout DF level of 1000 or even higher. This ultra level of DF doen’t correlate with better sound quality.

There are speakers that sound superb driven by amplifiers with a DF of 2 or 3. It all depends on the speaker design mandate.
Charles

Pairing of speakers with amp is really the key to any system. It's not just efficiency and power, it's clarity at desired volumes, voicing, damping, etc. I've always been of the opinion that you choose, in order: room, desired listening volumes, then speakers, then try different amps until you understand what the different amps are doing, then choose your amp. Bass in-room is the downfall of many an a'phile, most of whom cannot recognize standing waves when they hear the effect. Many audiophiles cannot hear when speakers are wired out of phase!