Any real experiences with a Luxman C900u and C10x?

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.

+1 @ghdprentice on the sound attributes of Luxman.  I extensively demoed one of the Class AB Integrateds with Persona 7F speakers.  I was all set to take them for in home trial, but was disappointed in the harshness and lack of midrange warmth.  Unlike @ghdprentice, I drifted to the much more musial SS products from Audonet.  I find Audionet very much more musical and delicate, yet full and dynamic.  My thoughts continue to be that if I can get this sound without tubes, it is the way to go.

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.

Also, not sure where you are located but I would suggest giving Albert Sportis at Precision Audio/Video in Chicago a call.  He is a pretty large volume and experienced Luxman dealer and may be able to help you with a comparison?

Thank you @wswright20