If you can try it first do, if that is not an option go ahead and buy it but keep your ARC pieces so you can compare and sell off what you don't like because it could be a combination of the two brands which do work well together.
I have a Pass X250 and really like it and yeah I hear how much the newer models are better but I enjoy my X250 and it sounds great on my current system so I have no desire to move up yet. That being said I have never heard the X150.5 but know your 100.2 SS amp because I used to own one and I really enjoyed that amp but it isn't in the same league as my X250. Don't get me wrong the 100.2 is a special amp but it's not close. If the X150.5 is as good or better than the X250 then you will be more than happy and never look back.
As far as your ARC LS25 MKII goes, I was fortunate enough last summer when my local ARC dealer picked one in on trade I took it home to see if I would like it over my ModWright SWL 9.0 SE and I didn't. Nothing wrong with the LS25 MKII but it was not as musical as the little MW... not even close to my ears. At the time I still had the 100.2 so I did have your same set up and it sounds really good so not at bashing it. I ended up selling my 100.2 for the Pass and did end up selling my SWL 9.0 SE for a MW LS36.5 which is definitely a big step up from the LS 25MKII and SWL 9.0 SE.
I've never heard the Pass X2 so I can't comment on that but no doubt if you want an amp that is more extended, cleaner and still has the magic of your 100.2 the X150.5 will have it. I'd be surprised if the X2 is not a better preamp though but no experience with it.
I assume you are running balanced IC's on the ARC? If you are not you should try that first and if you do pick up the Pass pieces definitely run balance there.
Good luck
