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

Current amp great soundstage but not exciting.
I find nothing wrong with a dealer or manufacturer making a contribution to a thread if they feel it’s warranted. Even if such posts might be perceived as blatant self-promotion, it’s up to the OP to evaluate the content of each post, as well as the reasons why such info might be discounted. Most here are adults, so I’m confident that everyone can discern the content of each post for what it’s worth.
@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.