RF/electromagnetic "pollution" comes to digital audio signal from cheap switching power supplies, WiFi circuitry in close proximity to DAC chipset. High quality streamers have properly shielded internal boards and best ones have all major components galvanically isolated with fiberoptic cables.
Source material is important, eletronics , power supply, cables, everything. I use Shield for music and video but I'd rather stream music from a dedicated streamer like Bluesound Node and use Shield for video only.