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

Current amp great soundstage but not exciting.
And I’m sure the OP (or any other causal reader) can discern that, too. You seem to discount the power of endorsement by an owner having spent hard-earned money on their gear. 
Quite the contrary, I do listen to those who have spent their hard earned cash on a product - like I have.  

I do not however trust a salesman who just picked up a new line that is suddenly the end all be all now that they carry it.  Where was all the praise for Krell before they could sell it to me?  That's where my BS detector goes off.
@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. 
An owner endorsing gear they own is simply bias confirmation. That in of itself should trigger a red flag in the minds of prospective purchasers. Are such endorsements valuable? Sure they are. Are they more valuable than anyone else’s input? I doubt it. 
An owner endorsing gear they own is simply bias confirmation.

@celander   I get your point, but what do you recommend?

It's a journey for most of us that [hopefully] moves our understanding and knowledge forward. Sharing where one is, at that point in time, and the choices made along the way [for better or for worse] seems reasonable and fair.