Which preamp for a pass labs?

Pass, they are made for each other....Super reliable.

Go for the XP-10.  Synergy is everything.

Alright, I'll go against the grain and say ARC. I am one of many folks who use ARC preamps with Pass Labs, and I am very happy, I think they blend very well.
The Pass Labs house sound is very full and warm, while the ARC sound is high resolution.
In my opinion, all ARC is too bright for my tastes, and all Pass Labs is too dark for my tastes. Together, the balance is just right.

Obviously, others will have other opinions.

I ran an LS25 MK1 into an X350 and thought they went together really well. I now use a Ref2 with the X350 and again a good combo. It's like McGrogan said, they complement each other very well.

The LS25 is now running an ARC VT100 and the sound is much leaner than it was with the X350. I'm still trying to work that out. It could just be the VT100 is just getting tired. I bought it used.

I will admit I've never tried any of the Pass preamps. It probably wouldn't be a bad choice. The LS25 does have an incredible amount of input/output options so that might also count for something.