High-efficiency speakers and high-power amps?

Hodu,
There'd be no harm but for sure the sound will vary and likely significantly. The amplifier-speaker interaction determines the sound, low power tubes yield a different presentation from high power solid state. Which is better is up to you. With very sensitive/efficient speakers you're often listening to only the first watt of power or just fractions of that single watt. Try it and listen.
Charles,

As long as you are careful it will be fine. You don't want to blow the drivers.

But the main reason people use low power amps is because they tend to have less amplifier stages. This can mean a purer the sound.

If you have a push-pull amp (unless its one of ours) the distortion will increase at a certain minimum power level maybe 5 or 10 watts) and then *increase* at power levels below that.

The distortion is audible and obscures low level detail. This will *seem* to make the larger amp seem less detailed, and happens whether it is a tube or transistor amp.

This is why smaller amps are often used on higher efficiency loudspeakers.

SETs and our amps are built in such a way that distortion decreases linearly to zero as power is decreased. This makes such amplifiers a good match when used on high efficiency speakers. You might often be using less than a watt most of the time- and with no distortion (or so little as to be unmeasurable) the result will be greater transparency and 'magic'.

Of course its not really magic, its just physics :)

Hodu,
As you see Atmasphere made reference to the concept of using 1 watt or often less than 1 watt of power. Just as he said you want to use an amplifier that decreases distortion at these very low power levels. This will result in a sound that's more natural, pure and as a result more realism with less artifacts. These very efficient speakers provide good volume and dynamics with tiny power demands from the amplifier, it's about sonic purity.
Charles,