I have the same set up you are considering Axle. I think it sounds great. I purchased the ATV specifically to stream via HDMI so I have never tried it direct via S/PDIF but I have streamed directly from my Macbook Pro via optical and the ATV-HD621 is in a different league IMHO.
Set-up isn't too difficult if you are MConfig literate at all.
Set-up isn't too difficult if you are MConfig literate at all.