Recommendation for a highly resolving amp

This one is super easy to figure out!
If you want to be 100% sure it’s the amp, connect your Lumin directly to the X250.5 using XLR or RCA interconnects and use Lumin volume control.
My money’s on the preamp as a culprit. 
Pass Labs amps and preamps are very transparent and resolving. I used to own Pass X250.5 and I am pretty sure the amplifier isn’t your issue.

Btw what cables are you using?

I am not sure what your budget so I can’t make a recommendation but I do not think you issue is the amplifier. Your pre-amp has a tubed output stage with 6SN7 tubes. What tubes do you have in there ? They can impact your sound significantly. You might try a Pass Labs pre-amp or other solid state linestage as well.

Other things to consider,
speaker set up.  How are your speakers setup compared to your friends?  How is your room setup compared to your friends?
Mine are four feet from the back wall and three feet from the side walls. Mine also sound better when raised higher off the floor.
 I know it might be a pain, but try a speaker swap with your friend.

All the best.

JD

JRDG 625 S2

I don't think the amp is your problem.   I would consider a different pre.

Respectfully, 
Jose