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.)