Ended up buying PHA-2 for my ZX-1 after testing it out with a number of different cans I have. I am still waiting on my Roxanne and JH16Pro (repair), so those impressions will have to come later.
My initial impressions:
1) AUDEZE headphones: PHA-2 can actually drive my large cans (LCD-XC and LCD-2) without major issues. In fact, at high gain, LCD-XC sounded almost as good as with my previous set up (ZX-1 - LO - RXmk3-b+). I felt that, although the PHA-2 did not pack the same punch and "meatiness" that RXmk3-b+ provided, there was definitely some gain in SQ from bypassing ZX-1s internal DAC. I felt that there was a lot more resolution with the PHA-2 setup, and also deeper soundstage. ZX-1 standalone, at least to me, has a wide but fairly "shallow" soundstage.
LCD-2 sounded a little thin with the PHA-2, compared to my desktop or the ALO rig, but still decent, with same positives regarding resolution/soundstage. I did not check all the specs, but I guess LCD-2 is a more difficult can to drive... I will probably never use the LCD-2 in a portable setup, so I was fine with this.
The ultimate setup for me was ZX1 - High Res cable - PHA-2 - LO - RXmk3-b+. I used an ALO mini-to-mini cable to connect PHA-2's LO to RXmk3-b+ and used Moon Audio Black Dragon balanced cable (yes, the source is not balanced, so not much to be gained with this...) The setup certainly sounded the best, taking advantage of the PHA-2 DAC while providing the punch of the ALO amp that I have gotten used to. The only problem is that I was left with another triple decker, which was actually one of the reasons I wanted to get a high-end DAP anyway.
2) "Mid-Fi" headphones: I have a Sony MDR-1RBT and a B&O H6. Both sounded fantastic with the PHA-2. Despite their relatively low power requirement, I actually thought ZX-1 did not properly drive these cans un-amped, and the PHA-2 dramatically improved SQ of both cans. I guess the B&O in particular benefited from PHA-2. Just beautiful bass - tight, fast and meaty, but not overdone - from the small cans. Un-amped, the B&O sounds very light overall, but it becomes a completely different animal with the proper amping it seems. Likely to be my weekend "travel light and/or Starbucks" rig for the next few months.
3) The only CIEMs I had on hand was JH5 Pros, which I use for my running, but I had to try them with the PHA-2. Again, once I get Roxannes and my "reconstructed" JH16s, I will post my impressions. First of all, no hiss or white noise with the PHA-2!!!! Since both of my currently-owned portable amps (the ALO and Fiio E12) are very hiss-prone with sensitive CIEMs, I was happy to find out that the background white noise was not present with the PHA-2 when I plugged in the JH5s. PHA-2 certainly enhances the SQ (again, resolution and soundstage) when compared with listening to ZX-1 standalone.
Just to keep everything in context, I think I will likely choose JH16s/Roxannes un-amped with ZX-1 for daily use. I will decide for sure when I get the CIEMs back from JH and give them through test-runs before full commitment, but the convenience of just slipping ZX-1 in my pocket when I am on a crowded train in Tokyo, even with all the aforementioned SQ gains, is probably hard to beat. I do agree with AnakChan that ZX-1 standalone with a decent set of CIEMs is a high quality rig that allows me enjoy music almost to the fullest extent. Certainly more so than my iPhone 5 or even the X3... Besides, it does allow me to blend in with the crowd a little better than when I have my LCD-XCs only already huge head, carrying a triple decker in one hand....