Pls help me decide Pass Labs X250.8 or Luxman M900u

OP,

Just to point to the reality that your own values and system are the most important aspects in choosing an amp. I completely disagree with @mtdining on every point he makes. This does not mean he is wrong for him. I have 50 years of experience in pursuing high end. I have owned Pass amps for over thirty years and spent a lot of time auditioning Luxman, Boulder, and Spectral. My conclusion is exactly the opposite. Pass exceeds in bass extension, and particularly rhythm and pace… making his amps far more compelling.

This is why you must audition amps in your own system. I am not doubting @mtdining has not experienced what he advocates… just that our values and systems are different.

OP - I also agree with ghdprentice. One particular point of difference ref. mtdining - Pass amps are very refined in the top end, which some may interpret as "a lack of high end". Unamplified acoustic music has an apparent lack of top end compared to most "hi fi" reproduction. It depends on what one's point of reference is.

Take this with a grain of salt as I am a dealer for T+A Elektroakustik, but look into the T+A A 3000 HV amplifier. It has the richness of a Luxman or Pass but will control speakers much better than either of them at the frequency extremes. It is stable down to 1 ohm and can drive the lowest sensitivity speakers. It is part of a modular system as well, where you can get a second and them into mono mode, or you can get an optional external power supply which separates the input stage power from the output stage power and give double its capacitance to 240,000uF. 

I use this amplifier as my personal reference and they pair wonderfully with Vivid Audio Kaya 90, designed by Laurence Dickie who designed the B&W Nautilus.

Happy to answer any questions should you have them. 

@ghdprentice ​​​​@yoyoyaya I’m with you. Not sure what @mtdining heard or heard of but my experience after owning 3 different Pass amps (X250.5, XA30.8 and X260.8) and running them with 8 different preamps (tubed, hybrid and solid state) and 5 different sets of speakers - dynamic and electrostatic, is that the Pass amps are as dynamic and transparent in bass, mids and highs as the signal you feed to them.  There is no lack of high frequencies or any difficulties in the bass. Excellent mids as well. 
Mismatch the impedance between a preamp you’re using and the amps, and you will get crappy, lifeless sound.
Find a synergistic match and you get explosive dynamics that will make the hair on your ⚽️ ⚽️ stand up.

Boulder amps start at what, $20,000? They better be better!

@OP  - audphile1 makes a very important point - it is critical to listen to the pre power combination, even moreso if a tube pre is being contemplated.