My friend, this is why those equipments are too cheap... es because the after-sales service is awful. I always wait some answer from the manufacturer defending the product, but I think the goal is to sell only.
Your problem is very simple, I don't know why some technical expert from Line6 didn't replies you....
You need to connect your head-amp to a powerline with ground... because the electronic circuit needs to drain some electricity when is turned off. I had the some issues as you and I resolved as I said.
Try to connect your amp to an electrical plug with GROUND connection.
Simple as water, difficult answer for Line6
BYEEEEE