Which would you choose; Pass 250.8, arc 75SE or luxman m900u?

Current amp great soundstage but not exciting.
@guyl, ARC and others design their gear to work synergistically together. So I’m not surprised you enjoy your ARC combination.
@celander You are right. Impedance matching is particularly important and I've learned the hard way that mixing components that are great on paper but do not mix well does not provide the optimal setup. Of course, great combinations can be found even when components are not from the same company but like you said, gear coming from the same place is expected to sound great. It also help 'solve' the "what if" problem as what you get is what was intended in term of synergy and you are no longer wondering how the system would sound if you were to change a component in the chain. At least it worked for me as I do not have the same sense of urgency to make a move as before. If I wanted a different sound, it is very likely I would have to change the whole setup and go a different route. Anyway, some people may disagree with my statement but for me, finding a combination I like helped me focus more on listening to music than to my components/system :-)
For the even more enlightened, taking a thread completely off topic and steering it to products you represent that the OP never had ANY interest in - will result in readers criticism... and rightly so.

This crosses the line between providing information and being a D-bag.
@guyl, +1 for your impedance matching insight. I have a new thread devoted to that topic (again).

Yes, manufacturers often voice their gear together. Once, the late John Ulrick of Infinity Systems and Spectron Audio fame told me to keep my Dunlavy SC-IVa’s with my Spectron Audio Digital One Class D amp because John Dunlavy voiced his SC speakers with that amp as one of his go to amps. 
@guyl - good to know you found the move from X250.5 to Ref 75 rewarding! Perhaps I should consider that move. I spent so much time talking to ARC and Pass before I made the decision to pair my Ref 5 SE with the X250.5 I was initially discouraged by the 20K min required i/p impedance that ARC expects. They pretty much said that thats a statement that isnt necessarily valid anymore. Apparently the ref 5's o/p impedance doesnt swing much beyond 1500 ohms and a 20K i/p imedance on the Pass was more than sufficient. Curious - what improved when you went from X250.5 to Ref 75? Thanks!