I currently have the C900U with mono M900U amps driving Focal Scala EVO's with a stereo pair of Rel 31's. Have moved from Mac with B&W and SF in the past and greatly prefer my current setup. I have found it to be extremely cable sensitive however! I stream almost exclusively with Tidal through an Esoteric N-01XD. I was able to listen to both the C10X and the M10X at Axpona this year with Magico and decided that I would have to do a direct comparison in my room to determine if there is a significant enough difference to justify the upgrade. For what it's worth the Luxman reps state the C10X to be the more significant of the two upgrades. If it were me, given the attractive pricing for the C900U on the used market, I would consider giving one a try. Likely not a significant financial risk even if you decide to eventually move to the newer model.
Any real experiences with a Luxman C900u and C10x?
I'm in the market for a new preamp and have the M10x and am leaning to the Luxman C900u or C10x. There aren't many dealers around me that have both for me to try and there doesn't seem to be much info on both online (i may not be looking in the right places 
)