I am running a Pass X250.5. After trying all sorts of preamps, I wound up purchasing an H2O Audio Fire, but I've actually never seen one up for sale used. I have a Pass XP-10 coming next week, which are sometimes available for around 3k. I can let you know how it compairs once I get it in the system.
I wonder how many people who are recommending passives have actually tried them with the X250.5. I wasn't too happy with the results when I tried a passive.
The H2O Fire will allow a firm grip on the bass but will remain very transparent (in a musical way) overall. It has less glare in the upper mids and treble than the Audio Research LS-26, but retains a fuller midrange than any other solid state preamp I've tried.
It appears you are only considering solid state. The Cary SLP-05 also sounds great with the X250.5, but is a little rolled off comparitively in the lowest bass.
Alan