Is powerfull Amps only for low sensitivity speakes?

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!

Kalali 1-20-2018
The OTLs are exceptions because they tend to have an un-tube like very low output impedance thus a higher than average damping factor.

Hi Kalali,

This is a rare instance in which I must correct an oversight in your usually sage commentary. OTL power amps do NOT have very low output impedance. A fundamental purpose of the output transformers that are incorporated into most tube amp designs is to transform the typically very high impedance of tube-based circuits to a much lower output impedance, which is accomplished by stepping down the signal voltage provided by those tubes while stepping up the current. OTLs typically compensate for the lack of the impedance conversion a transformer would provide by using a plurality of output tubes, and by choosing tubes suited to operation at relatively low voltages, but nevertheless the resulting output impedances and damping factors are usually in the same ballpark as those of many tube amps which have output transformers.

Atma-Sphere OTLs, for example, have damping factors ranging from a bit more than 1 to around 5 or so. An OTL that used to be made by Tenor had a higher damping factor, around 20, but even so the recommended load impedance for that amp was 8 ohms minimum.

Also, speaking of sage commentary, +1 to everything in the post by Charles which followed shortly after yours.

Best regards,
-- Al