I took half an hour composing this reply so it wouldn't look terribly insulting or as a challenge to you, but I can't seem to make it sound as diplomatic as I feel. Another career that I need to avoid... Sorry. I hope you understand, I really want to know why you would observe what you have. perhaps you meant to say "low-level" detail and not "low-end" detail?
I'm not trying to "bust your chops here" with my post, but I really find this confusing. If you are using digital out from Mac vs digital out from Wadia, with the same source files and the same external DAC, I don't understand how that is possible to get more low-end detail. Not unless your optical cable out of Mac is dropping data bits vs the $30 coax which is not, which can happen because my understanding is that digital-audio is one-way and does not use checksums to guarantee bit perfect like with hard drive data and such.
But, I always thought your digital cable would have to be exceptionally bad for dropped data bits to happen. I would expect dropped data-bits to similarly decrease the resolution in the high-end more than the low-end. I would guess that since low freq are longer sound waves, that there are more data points captured at 44.1Khz and losing one or two data points to a cable should have less effect in the low-end vs high-end. I am not an expert, so I welcome anyone's feedback about this phenomenon.
Basically i'm hearing more detail from tracks especially from instruments and vocals in the background. They are clearer and more prominent than when using optical from the mac.
For example at the start of summer of '69 ( waits to be flammed ) there is an underlying guitar chord that i can hardly make out via optical, its there just very far back and almost drowned out, whereas when i listen to it via coax from the wadia it's crystal clear and perfectly separated from everything else around it. Its this type of extra detail i was referring to as i low level, im sure there's is a more technical name for it.
I guess it could be due to a number of things such as the wadia being better than itunes at handling music, difference in cables, coax input better than optical who knows. But it's sufficient enough of an improvement for me not to want to go back to optical anytime soon.