I bought a Topping TP10 amplifier and an Omnitronic LH-015 mixer.
The idea was to mix the output from my TV with the output from the Airport Express and play it through my old but perfectly good speakers. Not to hear both at the same time, but to have a permanent invisible setup not requiring yet an other remote control.
The amplifier is a class D digital amplifier which is very power efficient, the idea is that I don't need to turn it off as it only use 1.5 watts when it gets no signal.
However I can't completely recommend the TP10 for music as it is more demanding than TV audio. It is just a bit too weak to drive my speakers.
So now I am looking at something better, but without going to a full size (and stand-by power hungry) amplifier.
The kickAmp on Amazon sounds like what I am looking for. However 1 review and almost nothing on Google is not great.
http://www.kickamp.com
There are plenty of class D amplifiers for cars, but they are at least 1000 watts which make no sense in a book shelf.
The trouble with amplifiers is that the specifications don't tell the truth - certainly not for the automotive ones.