Ok so my Mini to Toslink cable arrived. Plugged in to the Macbook Pro headphone port, turned the knob on the X-DAC to Optical and bham, worked without a hitch. No need to select it as the output in preferences. Only does up to 96kHz mind, not the full 192kHz due to the Macbook Pro limitation.
On to the comparison...
USB vs SPDIF with the X-DAC via a Macbook Pro.
Now I know, in theory, they should sound exactly the same, because they are both digital, but unless I'm going crazy, they sound different to my ears. Maybe it's the way the Macbook Pro sends out the information, maybe it's the way the X-DAC collates or processes it, but after doing numerous A/B comparisons, I'm pretty sure they do sound different. So what are the differences that I can make out?
Firstly, from the USB the sound is ever so slightly sharper and more airy. I also find the soundstage slightly more open.
I do this thing when I'm listening that makes me look like a crazy fool, but I visualise the sound stage with my eyes closed and use my hands to show the width of it, then when my eyes open, I make a record of it's size based on markings on the wall correlating with the location of my hands. In this instance, on every single A/B track tested, USB came out on top with more width or space for whatever reason. Now I know soundstage might not necessarily exist in the scientific sense, my prediction is that soundstage is often a product of the balance of different frequencies. From listening experience for example, I find a dip in low mids increases the illusion of sound stage, and excess sub bass decreases it.
In any case, through SPDIF the sound is slightly more intimate. Whilst I can't be certain, I think it might be due to a slightly more natural or softer sound, a tad more sub bass and ever so slightly more forward mids. In this sense, whilst the two are extremely similar, I'd say the SPDIF connection sounded a tad more musical or analogue like (despite being digital), and the USB connection to be ever so slightly more detailed and spacious, at least plugged in to the Macbook Pro anyway. That said, they both have pretty much equal macro detailing, it's just a smidgen easier to make it out with USB, but the hairline details are definitely there with both. You could say that USB just has that touch of sheen whilst the SPDIF goes for a more natural gloss.
Which do I prefer? Hard to say. It's awesome that I have the choice to do both though. I just leave both plugged in to the back of the X-DAC and connect to the Macbook Pro depending on what sort of sound I desire. Being a detail addict, at this point I'd probably give the nod towards USB, then again, I'm really loving the naturalness of the SPDIF. I'm enjoying both at this moment and time. I have no doubt some will prefer SPDIF for the musicality of it. But only if any of you are mad enough to consistently A/B compare like me or even notice a difference.
I'm sort of secretly hoping someone else will do the comparison as well to confirm my findings lol.
If anyone's interested, this is the Mini to Toslink cable I'm using.
Fisual Pearl Mini to Toslink.
Edited by Naim.F.C - 2/1/12 at 3:25pm