Suitable tube preamp for pass labs x250

Definitely go with a true balanced pre-amp. I ended up with a BAT VK3ix After my search for a pre to match my X150. I liked it better than Pass's own X2.5 pre.

Atmasphere or Sonic Frontiers?

When I had my Pass 250.5 I used it with a Thor TA-1000, absolutely superb match.

I would pay attention to what Tvad said. Great preamps won't sound so good if not matched properly with the input impedance of the amp. One fine tube preamp that can qualify would be the Joule LA150-MKii with the MU follower engaged (400 ohm output impedance set like that), should be able to drive most any load. Though it does not run balanced. THe Atma-sphere MP-3 is also quite good, but I'm not sure what the output impedance is - I think it is 600 ohms, which should work fine.

I used a Cary SLP-05 with my Pass Labs X350.5 Amp and the combo was not good.
There was limted frequencey on the ends and low dynamics.

Suggest going to a Pass Labs Preamp.