Concerning the graininess I've been hearing, I've found it improves significantly when fed straight from my S9. I think the udac2 coming from my computer has a slight midrange-ish warmth and forwardness that isn't a great combo with the already forward mids of the 598s. This makes sense to me as I mostly noticed the grain/distortion in the midbass area (such as loud drums on accentuated downbeats). Also, turning a bit of BBE on seems to cool things down more and improve detail (as much as most people here snear at sound enhancements, BBE really is great when used judiciously).
So, is the problem with the udac's DAC section or it's amplification? That is, will a cooler (and better quality) amp improve the SQ that I can get from the udac2/598s or is the DAC doing the damage?