I did something similar last weekend. I initially had the cloth side of the sub facing the wall. My thought was the sound waves were bouncing off the wall and right back at the speaker causing the distortion, since mine sits close to the wall.
So, I flipped the sub around so that the cloth side faces me (the couch) and so far so good. I still don't know why this never happened while using 4K disc, but all seems well now so I'm cool.