I don't know if it's to late to reply, but . . .
As @jpm asked, does anything sound good going into the USB input of the Ayon? It could be that the USB input is just not that great. They vary widely from DAC to DAC.
I doubt very much that money spent on a fancy USB cable is going to make a significant difference to sound that is fundamentally unsatisfactory. I wouldn't do it, if I were you.
You could try a Mutec MC-3+USB reclocker. It breathed new life into my Classe CP-800, really improving the imaging stability, focus, and life of the music. It would take the Melco USB, reclock it, and then output SPDIF for your Ayon. This unit has rave reviews in both pro audio and audiophile publications. Get it from somewhere that will take it back, so you're not risking anything.
Hope that helps