Which preamp to go with pass labs x150.8

…but an Ayre KX-R is fantastic also…. RIP Charlie

I do not suggest a tube preamp with any SS amplifier,as it is not a good match.
 

There are many including myself that would strongly disagree with this!

For Passlabs  branded preamp, I would look at Xp-22 or higher. Passlabs amplifiers are already dark sounding, so I would recommend matching them with the most transparent preamp you can afford. You already have a relatively smooth sound with the pass lab amps, so I would recommend matching it with a solid state preamp rather than tube, otherwise you risk losing details and dynamics

I owned an Audio Research Reference 5SE for nearly ten years… marvelous preamp.  If I had not upgraded the rest of my system and in the process my ARC to REF6se, I would still be very happy with it. For much of that time I had a Pass x350. A very happy combination.

@panerai557 Your amplifier has balanced inputs. If you want to get the most out of them, the preamp should support AES48 which is the balanced line standard. To the best of my knowledge, ARC has never made such a preamp despite being balanced.

An important tenent of AES48 is that the ground (shield) isn't part of the audio circuit connecting the preamp to the amp. Its only a shield. The actual audio signal is a twisted pair that's inside. This helps give you a lot of cable immunity as well as ground loop immunity.

There are tube preamps that support AES48 FWIW. I would widen your search.