)I actually had that exact combination. I came from a Luxman integrated so probably same sound signature but it was definitely an improvement as you would expect.
I wanted to use one line stage / preamp for both my tube amp ( 300b PSet) and the Luxman M10X.
I have an Aric Audio Motherload preamp using some “BAD BOY” NOS 6SN7. That I compared with the C900 and I would not call it better but just a little more laid back as you would expect.
Anyway I ended up keeping the tube preamp.
I can now go between the M10X and the Aric Audio to change the sound from a more detailed sound to a hard-hitting sound and a laid-back sound of the 300b.
I would say using the c900 just gave it a little more detailed sound. I would imagine it would be hard to find a better solid state preamp to match with the M10X (except maybe new c10x). The prices of the c900 are really good now and if I totally separated my systems I would go back to it.
Focal Sopra #3 speakers for now.