My last few cable purchases have been DH Labs. I think the D750 Coax and Mirage cables are excellent quality for the money. I also have their Air Matrix Cryo RCA and my all tube system is dead quiet .... put your ear right against the horn tweeter and it is silent . Cabling has a lot to do with nice black backgrounds.
I use both digital cables in my system and really think any perceived difference is the quality of the DAC clock VS the source clock. You really don't know until you try, but don't buy a USB unless you think your system can exploit any small difference.. I think the logic for USB to the Pontus is that it's clock is superior. I find myself using Coax in my system but I know my source clock is better than my DACs clock... is it in my head? Placebo effect? Do I read too many USB VS Coax threads? Who knows
They do exactly what a good cable should do in my system, reject noise and impart no sound or seem to alter tonal balance.