so, first answer is:
Pass X2 better than ARC LS25/II (in the opinion of jl35's friend, it's understood).
I've read many comments about the X150.5 power amp, and they all told about a very good amp, but I did not find comparisons with my venerable 100.2.
on the other hand, I've found very few comments on Pass X2, with my surprise: a lot of comments on X1, X2.5 and others, but few on X2, so I really don't have the faintest idea about what to expect.
question: so, daniele, why are you thinking of that combo if you don't really know about it ?
answer: I've always been attracted by Pass products, I always thought I would try some soon or later, and I've found that combo at what seems an honest price to me :)
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.
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.
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.
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.
Another one here with an ARC Ref 2 Mk I with GNSC ref mod& Pass X350.5. Prior to the Ref 2, I was using an ARC LS25 Mk I with GNSC ref mod.
Had an opportunity to spend an afternoon at a friend's place years back when he was trying to figure out what to keep between ARC Ref 2 Mk I, Pass X0.2, Pass X350, Pass X350.5.
I've also heard quite a bit of ARC Ref 2 Mk II, Ref 3, Ref 5, Ref 5 SE, and several of the ARC amps.
Here's my take:
1/ There is a huge difference between Pass X and the X.5 amps. The Pass X amp sounds very cold and sterile, has the signature of your typical solid state amp. The X.5 has a warmer tube-like texture that Pass is well known for.
2/ Pass X0.2 preamp actually sounded more like a tube pre than the Ref 2 Mk I (with stock Sovtek tubes). But The Ref 2 allows you to tube roll to change the sound signature. The Mk II and the later ARC pre's using the 6H30 tubes all have a sound signature that's cooler and faster.
3/ Pass amps have great control of the bass, something that ARC tube amps just can't compete.
I like my Ref 2/Pass combo. If I didn't like the hassle of tubes, Pass pre would probably be my choice for a SS pre. If dynamic impact (slam) is not so important to me, I could definitely live with an ARC tube amp.
Back to your question, I think it's not a step forward, but a side-way move. You will probably hear improvement in certain areas such as extended and more solid bass, what you will loose is some of that midrange magic (fron 100.2, not from the Ls-25 Mk II.)