)I've had the M10X / C10X combination for 9 months. I didn't have a chance to compare C10X vs C900u though.
I use this combo with a Chord DAVE dac and Magico S3 mk 2 speakers. I was going for clarity, detail, and convincing musicality to the sound. To my ears- this combination brings that for sure! The sound is more LIVE MUSIC with little bloat to the sound. Instruments sound real, in particular ride cymbals (I'm a drummer so that was a goal) - Of course, this is all very subjective.
I did demo a 509X in home for a few days. Comparatively, this is cleaner - with more bass extension, better separation of instruments, and a bigger soundstage.
If you like the Luxman sound - the C10X is excellent.