After a few days of testing I have decided to return the Stereo90 and keep my transistor-based amplifier.
The main reasons are lack of power and sound coloration. Sound coloration is so nice on some types of music but I like a more neutral sound when it comes to dynamic. Pink Floyd (Another Brick in the Wall) or Dire Straits (Private Investigations) with the Stereo90 is not as punchy as with my other amplifier. It just sounds too nice ...
Anyway, it was fun to finally try out a very good tube amplifier in my setup. The design is beautiful but the sound comes first ...