having the same issue with my marshall jcm 2000 dsl contacted support and this is the response i received :
Thank you for contacting us! I have not yet hear d of an issue like this with the HX. Can I ask, have you had a chance to see if you get the same behavior? It sounds as if you are running into a ground loop hum.
wrote a response to the one above and never heard from them again!!! I find it hard to believe he's never heard of this issue this forum is proof in itself amongst several found on the net.
They have an issue but don't want to admit it. I found an HX user on the net which used a hum canceling box made by Morley Ive purchased one waiting for it to come in hope it cures the issue as it did for him in the video he posted with no thanks to line 6