Best 6NS7 Preamp for a Pass Labs XA25 amp

+1 on the Atma-sphere.  Surprised it hasn’t been mentioned yet.  I’m slumming it with an MP3 (kidding) but had tried the MP1 and was outstanding.  That’s what sold me instantly and went on a search for a MP3.  
Works great with my XA monos

You can't go wrong with a Supratek preamp. I have a Supratek Sauvignon (highly modded) mated with a FW SIT-3 amp and the pairing is wonderful!
The Supratek is the only component that I have not changed over the years. It's a keeper!

Aric Audio is great stuff, also Linear Tube Audio Preamp, MZ3 or even the MZ2 would be great.

Dehavilland Ultraverve, I used one for a few years. Excellent open, airy sounding preamplifier.

Another vote for the LTA preamp options. I started with the MZ2 with an XA30.8. Very good combination. Moving up to the Microzotl Preamp resulted in a meaningful improvement. Now this preamp is by far the most affordable piece in my system. I’ve never considered it the weak link. Real dimensionality, not just tube generated staging anomalies. Dynamics, harmonic completeness, and black see the Big Bang type of noiseless background. An absolute bargain.