I use Astro A40 TR's (gen 4) along with the mixamp, and i found that whenever i connect both my pc and xbox simultaneously there is a loud buzzin noise. I use HDMI from xbox to PC monitor (video) and use USB from xbox to mixamp.
On the pc side I use DVI to monitor (video) and use aux ports to mixamp for my audio.
I found that whenever both my HDMI and AUX cables are plugged, it makes this buzzing noise. But as soon as i remove either one, the buzzing stops.