My ZO2.3 just arrived! I probably won't write much about sound quality aspects after initial testing as last time it was such a big difference after burning it in a little.
ALL YOUR ZO ARE BELONG TO ME
What I can tell quickly is that the contour levels have definitely been altered, in a good way. Now it sounds balanced at the lowest setting with nothing exaggerated and the following low levels are also providing less bass boost than previously so now you can add a more fine level of bass boost! (there's at least 2-3 more finer levels of bass boost than on ZO2.1) I find myself using about 3 - 4 levels higher with this non-burnt in ZO2.3 than I used with the burnt-in ZO2.1 so seems the contour levels are much better balanced now. Should be nice for all those non-bass heads out there, you can add just a very tiny bit of boost or none at all. Thanks digiZoid for taking our feedback so seriously!
I don't think either it really sounds that bad using the low gain mode while using headphone-out jack. No increased distortion noted despite having to up the volume a little (in my case to 53% for my 64 ohm headphones compared to 16% with high gain). The bass boost will be giving less of a drastic change in low gain but it doesn't sound that bad at all but yea the bass is definitely stronger with high gain. I see it like an alternative bass contour shape in case the high gain doesn't work nicely for you, it's more smooth and definitely provides a huge boost also in low gain mode at the higher levels. Sound signature wise from very intial testing, reminds me slightly more of ZO1 than ZO2 but it's not sounding exactly like either of them. But more on that later but at least I seem to enjoy this ZO2.3 more on the first listen than I did with ZO2.1.
Also now it switches on/off MUCH faster and is almost completely distortion/noise free for me (well as good as it gets anyway). But the bypass feature is removed as I already knew so no suprises there. The hiss is reduced in ZO2.3 on high gain level compared to ZO2.1, testing with XB500 headphones which are among the most hiss-prone headphones around provided quite noticable hiss in ZO2.1 while it's very faint/almost inaudible on ZO2.3 and switching to low gain level which fits this very sensitive/easy driven headphone (probably IEMs too) and it becomes as good as silent, nice. This headphone brings normal listening volume levels with 12% in high gain and about 38~40% on the max level in low gain so it's probably wiser to use that as I notice this sensitive headphone also responds better to the contour levels in low gain compared to the M-Audio Q40 for example.
I don't use portable devices so can't test with LOD but that's my above experiences so far using headphone out. Quite positive impressions so far, the high/low gain certainly helped. I will use it for a while before going more in depth regarding sound quality aspects.