Is powerfull Amps only for low sensitivity speakes?

An additional point against using high powered SS amps driving extra efficient speakers is most SS amplifiers produce higher levels of undesirable (higher odd order) distortion within their first watt or two just like but not quite as much as when they are pushed near their clipping limit. The other related negative side effect of such pairing is the volume controls on most preamplifiers tend to be less linear, and more coarse in some cases, at the lower operating range and become more refined at the mid-higher range. 

+1 Kosst.

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.