I would also consider the SimAudio 740P preamp as a good option with your Pass amps. I'm using it with my Pass X250.8 and the two go together very well in my system.
I also owned the Pass XP-10 and Hegel P20 preamp with the X250.8 but the 740P was in another league (as it should be for the MSRP difference).
I was fortunate enough to find a nicely priced demo unit.
The LS36.5 was on my short-list to try, but once I tried the 740P I stopped my preamp search. Yes, it is not a tube preamp, but the Pass amps are already a bit laid back and throw a nice sound-stage, so coupled with a highly transparent+neutral solid state preamp is not a bad choice either.
I must say I was floored when I hooked the 740P up for the first time with my Pass amp :) The sound-stage and transparency of the two are amazing. I had tried the no preamp option for a short while (Berkeley Alpha Dac 2 direct into the X250.8) and the resolution and transparency was excellent, but adding the 740P added much more sound-stage depth and overall 3 dimension-ality to the music. The loss in transparency was very minimal if noticeable, more like just pushing the music further back, but the advantages were much greater. That being said, any of the preamps listed in this thread are definitely very expensive volume controls assuming you don't need switching like in my case (single digital source)...
Good Luck.