Which preamp to go with pass labs x150.8

depends what type of sonic signature you are seeking... everyone's 'best' is different

Audio Research. I have owned Pass amps for forty years and Pass preamps. The best mix to me (assuming you can;t afford an ARC amp) is an Audio Research preamp with Pass amp. I owned a Pass x350 for nearly 15 years.

Audio research old model amplifier is not expensive compared with pass amplifier.

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

Looks like Arc pre is the way to go? What about Ref 5 or 5SE? I know the particular model loved by many, but is it too old buy in the used market?

a REF5 or better yet an SE is a formidable preamp…. of course, i am biased….i have 5K hours of happy listening in w mine…..

Best to you and good luck on your musical journey