TL;DR, I was fortunate enough to order early and get the AMP4S and I'm very happy with it. Increased soundstage, better bass (to me), and more fun. All without losing the details I love from AMP1. And yes, that's completely subjective.
I thought I'd post a few comments on my experience with the AMP4S so far. I've had it for less than a week and probably only have 10-15 hours on it. I have one DX200 and have not been switching back and forth between AMP1 and AMP4S. So this is really just a first impression based on a lot of time with AMP1 and a few hours with AMP4S. And of course, it's pretty subjective.
Previously:
DX200>AMP1>CB12>IT03
Now:
DX200>AMP4S>CA01>CB12>IT03
I realize there is also the addition of the CA01, but I think the only real audible comparison for me should be between AMP1 and AMP4S. I'm sure some may hear a difference with and without the CA01, but I don't suspect I could.
I've not heard any of the other AMP models so can't comment or compare. So far I've only listed with the IT03s, but I did just get a new cable for my Oppo PM-3s and will try that soon.
The first thing I noticed was more bass presence. Not presence like the knob on the front of a Marshall, but presence as in more readily available. I'm not a basshead for sure but appreciate good quality bass. I think this is improved over AMP1. I admit it could just be better quality bass versus quantity.
The next thing for me was the soundstage. It's definitely expanded and everything just seems to have a bit more breathing room. Several times I noticed longer decay on notes or passages that I hadn't recognized before.
The other comment I would have is even more subjective than the others and will probably raise some eyebrows. I don't feel like I've lost any of the reference type qualities of AMP1, yet I feel like AMP4S is more fun. I think it just sounds more lively. Everything I've listened to so far just has a bit more...something. It's hard to describe. Some might say warm, and while that may be true, that's not what I'm trying to say. It feels more alive maybe.
Aside from the different physical connection possibilities, I don't feel like I'm missing out on anything at all with AMP4S.
Some of the songs I focused on, and my subjective thoughts:
Symphony of Destruction - Megadeth
One of my go-to tracks for heavier guitars. At first listen, I thought maybe the intro guitars had lost a little bit of their edge. Not the case. The bass is a little more prevalent I think and is a little more forceful but the guitars still have that impact.
So Much to Say - Dave Matthews Band
I like this one for ambiance and soundstage. Lots of details coming in here and there. Once again I feel more bass here as well as hear it.
Sultans of Swing - Dire Straits
A standard for critical listening for a lot of people, I know. But the subtle touches of Mark Knopfler's playing really stand out. The slides, multiple string bends, and vibrato are as apparent as with AMP1 but seem to have more room to shine here.
Medley: You Don't Love Me / Soul Serenade - The Allman Brothers
Another good one for ambiance and trying to get a sense of the space they're playing in. As with most of the Allman Brothers' stuff, there's a lot going on. The Soul Serenade break at around 9 minutes in is very well represented. Almost feels like being there.
Blue Sky (from S.U.N.Y. at Stonybrook) - The Allman Brothers
I choose this one partially for selfish reasons (11 minutes largely consisting of Duane, Dickey and Barry at their finest), and because it's a relatively rough recording. It's nice to know how forgiving the setup is. The shortcoming of the recording is here for sure, but not to the point where I wouldn't listen. In fact it's better in some ways as the basslines here are a third melody and stand out a bit more.
Suspended in Gaffa - Kate Bush
Not for everyone, maybe, but I'm a big fan. This one is a good barometer for sibilance and sensitivities at the higher registers. Moments of Pleasure is another one. Both sound great and really highlight the subtleties in the vocals.
Not that it makes a huge difference but these were all in Mango. I've been having some issues with ghost swipes and right now Android isn't usable for me. I think I probably just need to reinstall the firmware (2.98.198-L1).
With such a limited run of AMP4S (at least so far), I'm not sure if this is helpful to anyone or not. But if you can get your hands on one, I don't think you'd regret it.
Once again, this is just what I hear with AMP4S compared to AMP1 and is only my opinion.