from ARC to Pass ?

I've had the ARC REF-3 since it was  new. When everything else is right in your system, there's no need to look any further. 

First, I thank you all for your kind and interesting answers.
I know ARC Ref. 3, I often hear it play at a friend of mine's home. but here in Italy, where a live, I can find it (used) only at a much higher price than X2+X150.5 (say 4500€ vs 3500€).
One question: some of you advice using the X150.5 with a tube pre amp, maybe with an ARC preamp: I know X150.5 has a low input impedance; isn't it an issue with (most) tube preamps ? I guess it should be with ARC preamps, which 'love' high input impedance power amps.

Most ARC preamps won't handle less than 30K input impedance. So you can just barely pull it off, as the Pass specs do state a 30K input impedance. You might contact them and clarify what the 30K/20K spec is regarding their input impedance, just to be sure.

guess 30k = balanced and 20k = unbalanced

I get fantastic results with my Pass x250.5 mated with the ARC Ref 2. Since inserting the Ref 2 into my Pass based system I'm so happy with the results I haven't felt the need to look anymore.