After a lot of research, concerning the acoustic guitar amplification, I chose the K&K system, with the trinity mic system (3 sensors + mic), together with this preamp.
The unit is metallic, rather small, well built, easy to understand and deal with. It has lots of output possibilities, and it's possible to have several outputs working at the same time (for instance a general blended output for amplification on the stage and two separated outputs for recording).
It is very silent and the amplification is quite good.
The only thing that is not 5 stars / 5 is the phase inversion. It's possible to differentiate the two channels, but it's only on or off... For the stage, it might be fine. For the studio, a rotative selector, for fine tuning, might have been better... but it's alright.
Now, there's the question of the price, which is fairly high...
I suppose I would choose it again.