It's a strange occurrence, and it seems reproducible: http://forum.xda-developers.com/showthread.php?p=40911047&postcount=2
Basically, when I drive my UE600 at high volumes using the GS4, there is noticeable audio crackling. This mostly happens on the sub-bass volumes as when I use the equaliser to turn the volume of 1, 150 and 400dBs down there's no longer any audio crackling.
Any idea why this could happen? It doesn't happen with my other headphones, nor with the UE600 driven to the max on other sources.