What tube pre-amps pair well with Pass amps?

@billtro please excuse my short sidestep to your thread. Wish you the best in your search. 
Thanks 1markr! One day I may talk to Aric regarding upgrade. Love my Special as designed. 
One needs to know what one wants to achieve prior to throwing resources into upgrades. 

Boulder power amps and Focal speakers are a match made in heaven.
I use a Wavac pre amp with my 2060 but also looked closely at VAC pre amps.
Sonically the Wavac - Boulder combo is spectacular, but on paper there are better matched preamps for the Boulder

I’ve run my First Sound preamp with both the X150.8 and the X250.8 with great success.  Outstanding match in all sonic regards.  

Many years ago , when the X250 was in production, I used a Lector Z.O.E preamp. I think it’s an Italian audio company. Anyway, it sounded really good after trial and error with various nos tubes. I ended up selling it to “upgrade” but it was manhandled during shipping. So sad. Anyway, when my friends come over to hear my new gear a lot of them will compare the new stuff to the Lector/Pass combo , ie , xx isn’t as extended as your old setup. Sort of became a gold standard for them

Anxiously waiting my Nelson Pass Designed Korg B1 tube preamp!  It should be shipping soon: https://diyaudiostore.com/collections/b1-buffer-w-korg-triode-pre-amplifier
I heard one at his place in front of a SIT-3 and an F7 driving Tannoys.  Sublime!