Cesar, I have the Monitor Audio GR10s and thought they matched very well with tubes. Very detailed tweeter, but I felt it was a little dry. When I hooked it up to a 35W integrated tube amp the sound became much more enjoyable to my ears. A touch more warmth made them more friendly.
Placement can be tricky with them as I found the bass to be a little undefined. Foam plugs can be put in the ports to tune the sound, but the frequency response does get diminished. Still, for a bookshelf speaker they do a great job of recreating the low end.
