Recommendation for a highly resolving amp

The BAT 255SE amp is a fantastic amp, amazing value for money. I run 3 X BAT VK 655SE in one of my systems which has a similar sound to the VK 255SE but more power, but I don't believe your Pass Labs amp is your problem, if you feel it lacks some resolution.  Often pairing a tube pre amp with a solid state amp does not work well.  
You may wish to try a Pass Labs Pre-amp with you system but you may loose some warmth at the expense of detail and resolution. 
From my experience its always best to use amplification from the one manufacturer.
If you are seeking both warmth and resolution try a BAT pre-amp which is designed to work with BAT solid state amps and may work with the Pass Labs amps.  
Most panel speakers require warmer sounding amplification, I use Accuphase amplification, which has amazing detail and resolution without being analytical, with my Analysis Audio planar magnetic speakers.
Try and find dealers that will lend you the some equipment to experiment before committing.

The Don Sachs preamp is probably not your issue. I have this preamp...it's incredible.

If you took your friend's system and dropped it in your room, it would probably not sound the same anyway.

If the room is wrong, you will get time smear among other things.

The sounds seems to be really good until you reduce some smear and then you think, 'how did I think that was sounding good at all?' 

You really won't know until it's reduced. I say reduced because I don't think you can realistically remove all of it...nor would you want to...everything would sound dead. 

I now have dedicated AC lines, 15k more in upgraded components and the room in my old home kills this system. I was using an old Carver amp (although I too was using a Don Sach Model  2 preamp).

Is your room a dedicated listening room or a living room situation? If it's a living room, then you need to be clever about the acoustics.

Are you running your equipment on a dedicated power line? 

Some highly resolving amps I am personally very familiar with:
Benchmark AHB2
Bryston 28B cubed
First Watt SIT-3 and M-2

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?