from ARC to Pass ?

The Pass X2 isn't one of their better preamps.  I was even told this by one of the Pass employees over the phone.  Years ago I paired an X250 with the X2 pre & eventually went with a tube preamp which I preferred. 

+1 for the X250 or X250.5 with a tube pre amp.

As mentioned in my previous post you might want to try your LS25MKII with the X150.5. 

After I purchased my MW LS36.5 I pulled out the stock Sovtek's and went with EH 6H300Pi's from Cryoset Certified which really opened things up; highly recommend these tubes. 

If you're running stock Sovtex's or EH's you might be surprised what the cryoed EH's will do to your LS25MKII.

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.