from ARC to Pass ?

You'd probably be better off making the move from the ARC to a Herron VTSP-3A(r02).

http://www.cryoset.com/catalog/product_info.php?products_id=415&osCsid=95ddfef4fdc1d211879793363...

Sticking to the components you started the post with. No doubt the Pass is a more refined amp than your current ARC amp. Maybe just upgrade one thing at a time so you don't get confused or discouraged. 

Really not sure the Pass is a positive change over your LS25MKII, so maybe keep it for awhile and try some tubes and spend less for now.

Totally agree with others using a tube preamp on the X150.5. So maybe spend more later and step it up to a Herron, or other later.

Have to agree with the suggestion that you get the ARC Ref 3 (or above).  It is a phenomenal pre-amp, and I doubt you'll find anything for less than $6000+ that will sound as good.  Just incredible detail resolution, soundstaging and imaging.  I don't think you'll find many (if any) SS amps that will equal it.  

Team it with one of the Pass SS amps, and you'll likely be amazed.  

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.