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

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.

@yoyoyaya thank for the response , @audphile1  you make a very good point.

what specs do I need to look at to understand if the Backert Rhumba Extreme 1.4 would pair well with a Pass Labs X250.8 ?

https://backertlabs.com/tube-bias-adjusted-automatically/

https://backertlabs.com/more-goodies/

@garynospamm

I’ll chime in and I’m sure @yoyoyaya will as well…

few things to consider -

1. The input impedance of the amplifier should be at least 10x the output impedance of the preamplifier.
2. The amplifier input sensitivity and gain - pair a high gain preamp with a high gain amp and you’ll be losing some of the finesse of the preamp volume conttol as well as the volume range.
3. Figure out if you want to use XLR outs on a preamp - most preamps will double the voltage output on the XLR outs compared to their single ended outputs.

If you take the specs of the amp and preamp and reach out to their respective manufacturers, they’ll give you an idea if the pairing will work and or what aspects of sound quality may be compromised