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

I had the Pass 250.5 (not .8) with an ARC REF5 SE and sold it for a REF 75 which I still own. The ARC combo is the best I've heard in my room and I had a few high-end units over the years to compare (Bryston, Classe, Simaudio, Luxman, Pass Labs, etc). Synergy is critical and of course this is facilitated when components are coming from the same brand. I was skeptical initially about the relatively small power number of the ARC amp (75W) but it sounds as powerful as 200W SS amps I had in my room and does have the quality of tubes without the 'disadvantages' (very neutral sound not warm). It's a Stereophile class A if that can help :-) I highly recommend this ARC amp and I've read many times that it sounds better than its bigger brothers in the ARC line (could not confirm that myself though).  

@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 :-)

@pistha It's hard to describe but along with less heat (Pass amps are power hungry and I had to turn the thermostat down when the 250.5 was in system. no kidding ;-) ) the biggest change I heard is a fuller type of sound. Pass's midrange was sweet and clear but the mental image I had then when listening to my system was that there's was something missing, colors being washed out if I can use this picture analogy. Colors with the ARC were definitely fuller and more saturated in comparison (mid bass and midrange were definitely better). I could hear the synergy we are talking about. I guess one will get the same experience with running a XP10 with a X250.5. It's just that I had an ARC preamp not a Pass. Please note that I do not have a REF 75 SE. It's the regular version (not SE) but I replaced the KT120s with KT150s (big difference and part of the upgrade to SE). If you can get a factory SE, it's even better. Hope this helps, as I said it's hard to put in words all those differences but I purchased my REF 75 3 years ago and still remember my initial reaction when I hooked it up: "ahhhh got it!" I was -and am still - quite happy with my move.