I listened to a few tracks this morning with my entry level psb speakers to compare the inputs on the zodiac+ fed by squeezbox touch. To my ears, the differences between coaxial and optical were very slight to the point that they were hardly noticeable, or may be due to placebo.
The coaxial input with WW silver starlight cable sounded a bit more clear and crisp with greater bass impact, compared to optical with WW supernova 6 which was a bit more smoother and natural (less digital) sounding. Also, the volume of center of the stage for coaxial seemed a bit louder, while the volume across the stage, left to right, for optical seemed more balanced and I could here the ambient sounds a bit more clearly on opitcal. The observed differences were very slight and they may be due more to the difference in cable (sliver starlight versus super nova) than the difference between inputs, or due to placebo.
One difference that was clear for me was when I compared USB directly input from computer versus coaxial input via Audio-gd digital interface, which was connected to computer by USB. The sound stage was noticeably taller and wider with DI in the chain, and I preferred it over using USB directly.
Agreed on your staging comments between coax & optical, I hear it the same. I use the coax output on my Pure i20 dock and like it a bit more than USB coming out of my Mac Mini. I think I might have to get an AGD DI too, or a Hiface maybe.
I don't know of any dealers in the area. Your best bet is to contact Marcel, Antelope's rep for the US and see what he can work out. The best way to reach him is phone, I think I still have it so PM me if you want his number.