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

@mtdining can you elaborate? What are the signs the amplifier is not able to manage impedance dips?

@garynospamm if you end up with Pass Labs amplifier, consider Audio Research Ref 5SE or Ref 6 in your price range. Audio Research tube preamps pair extremely well with Pass Labs amps. Speaking from personal experience. 

Years ago I owned B&W N803 and drove them with Pass X250.5 and Audio Research LS-25 and Ref1 preamps. B&Ws love power and the X250.5 had zero issues driving them.
I now have Wilson Sabrina that dip to around 2ohms and have no problems with X260.8/XP22 combo. The amps, at normal listening levels below 90db, don’t even leave Class A.

Also, I had XA30.8 driving Martin Logan Montis and the Sabrinas. The MLs have nasty dip to below 1ohm if I’m not mistaking.  The XA30.8 didn’t care. It wasn’t as dynamic and didn’t have the control over the speakers the X260.8 possess with the Wilsons but it still had the midrange and treble magic. 
I have no experience with Luxman amps.

@yyzsantabarbara can totally see it with benchmark ahb2. 
it’s dimensions are 11.04" W x 3.88" H x 9.34 " D - Including feet and binding posts

and it weighs 12.5lbs

The power transformer in my XA30.8 was bigger than this entire amp.

Think quality vs. quantity and this is a case in point. 

@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!

@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 

Impedance is fine. 

@ghdprentice LOL I’m sure. I’m enjoying a Ref 6 now paired with my X260.8 monos, courtesy of my buddy who let me play with it for a week. Sweet combo!

@garynospamm …Ref 6 SE does have a processor feature but the Ref must be powered on to utilize it. If you use your HT a lot, using Ref in that system would just result you in shortening tube life by running the unit unnecessarily. 

You’re welcome.
You can configure “Processor” through the Ref’s menu and assign any input to it. Keep in mind Ref will turn that input into unity gain so HT processor is all you want to have hanging off that input.