It's funny that you should say that. I was initially only interested in M-900u, but I found your forum posts through my google search. Your enthusiastic opinions on the C-900u and a stellar review at Soundstage weighed heavily in my decision to purchase both.
I agree, operating the C-900u is a joy to operate without the remote control :D

Glad we could help enable each other in support of our mutual obsession! :music:

There is one thing about the C-900u I find very odd. When using the External Pre option with my Anthem processor, the signal is only sent to the single ended outputs on the C-900u. That I find to be a major drawback, because it means that the C-900u in addition to the balanced connection needs to be connected with single ended cables to the M-900u. When I am going to watch movies, I must always remember to toggle the M-900u to single ended mode as well.
I am sure that Luxman has some very particular reasons to design it this way, but from a usability point I find it very annoying.

Probably the same thing as with Accuphase... but the Accuphase PRE also has XLR processor input, that works with XLR out to the power amps. But if i use RCA from processor, i need to use RCA out to the power amps... my solution is to use a adapter from RCA to XLR, to the XLR processor input on Accuphase... but you have no XLR processor input :(.

Unblalaced
 

You can run a set of unbalanced cable and have line output set to both
I did this seems to work

Has anybody tried two M900u as monoblocks?

I am now running unbalanced into the M900u. I find it more dynamic and there is less hiss in the speakers. I wonder if it sounds better because the M900u is basically an unbalanced design? It is probably system dependent.:scratch2:
I set the C900u at a fixed volume when watching movies.