What tube pre-amps pair well with Pass amps?

I'll just note that the aric audio preamps are also 6sn7 based. I'd love to hear one, as every review is very positive (and I agree with many of his well-advertised design options).

+2 for the Aric Audio preamps! @laaudionut, @mesch 

Since we are on the topic of the Pass Labs XA25, I am now running a souped up Aric Audio Special 6SN7 preamp with mine, replacing the already excellent Backert Labs Rhumba preamp, which I used for 3 years.  This was Aric’s own preamp that he used to voice his latest Motherlode II preamp, and it retains most of the Motherlode’s sonic characteristics.  It’s an amazing match with my XA25. 

If Nelson pass wanted tube preamps, he would have made them  Stick with Nelson's designs. He is the best amplifier designer-engineer there is so why mess with tubes?

I will agree on his ability to build some of the best solid state amplification afforded the audio community. However, I have had both the XP20 and XP30 in my system(paired with the 60.8's), and they are anemic and lifeless in offering.

You may want a differential balanced preamp to bring out the best in Pass's balanced designs.  I use an AtmaSphere MP-1 preamp with XA 160.8 monoblocks and a modified XP-25 phono stage.  It sounds very clean and can be contoured to taste with tube rolling.