Their is something to be said for synergy.
I have a PASS X250 and tried both tube and other SS preamps. then bit the bullet and ordered the PASS X2.5. It worked perfect with the amp sounds great, has a remote, adjustable gain (4 and 12db) with balanced in and outputs.
The second best pre I had with it was a B&K pro10mc used in Passive mode....
I tried a Audio Research SP16 and took it out in 2 weeks..
My opinion stick with a PASS preamp you wont be sorry