I own M900u along with C900u. I also have Focal Sopra speakers that my friends told me would sound too bright with Luxman, mainly because of the beryllium tweeter. Room treatment is important as others have noted, but in my book the front end and property matched cables are even more so. Bottom line: look at your system as a whole and not just the two mentioned components to see what will sound best to you. Try to work with a dealer that will allow you to work with multiple set ups until you are happy.
If you decide to look at the M900u, mating it with C900u may possibly be your best option, as they were designed to work together.
Warmest, regards