Don’t worry.

As long as the sound is clear you’ll be OK. In home listening situations a 10W average is very loud for most people. Even though the average might be 10W (more likely it’s much less than that) there could be brief 100+W peaks for uncompressed classical music.

There are no hard rules used when assigning a speaker’s recommended power handling. I’m not accusing KEF of abusing the spec sheet, but typically it’s the marketer writing the spec sheet who assigns the number, with little or no input from engineering. Cheap, inferior speakers are often rated for high power in an attempt to influence potential customers who think that a high power rating implies a high quality speaker. Only the spec writer knows what the spec means.

Actually, low powered amplifiers damage more speakers than high powered amplifiers. If you give a 60W amplifier a 75W job, the amplifier will emit high frequency distortion products that will burn out the tweeter. Users who are habitually burning out tweeters need a more powerful amplifier (and some education).

We humans have been conditioned to equate distorted with “loud”. We raise the Volume until we perceive some distortion, then declare that the music is “loud”. For some listeners more distortion implies “louder”. In my college apartment we could run at levels where verbal communication was very difficult, but the sound was clear. Yet, we’d have people approach us, yelling in our ear with cupped hands, “turn it up!”, because it didn’t seem “loud”. That same crew in another apartment, using a little compact system operating at distress levels, was satisfied that the music was “loud” and conversation was easy.

I’m not familiar with these KEF’s, but some high end speakers actually dip below 4-Ohm’s at a few frequencies. This will stress a lot of amplifiers (they’ll sound “loud”) and they’ll become unstable or shut down. The SONOS AMP is one of the safest amplifiers available. It will tolerate occasional dips below 4-Ohms without complaint and it will shut down, rather than enter the high distortion mode that I described above -- thus protecting itself and the speaker. Unfortunately for many listeners, the AMP will never sound “loud”.

The most useful speaker specification for me is the sound pressure at one Watt, that lets you see how loud your amplifier will drive the speakers.

The Sonos Amp has enough power to drive almost any speaker loud enough to be acceptable.