Good point wid. Possibley, worth a try to gently wiggle and see if it could be confirmed to be the problem. If not, the issue still could be some component inside that the sub is vibrating but is not in direct contact with the YPbPr cable.
I had a cheap pair of speakers a long time ago (still have them actually that I use for Halloween). One of them quit working. Upon inspection it was a loose connection in the crossover circuit. One of the two metal leads of a capacitor broke. Not at the solder joint, but as the lead entered the base of the capacitor.
Try both the gentle wiggle test of the component cable, and also try moving the sub further from the TV, possibly to the back of the room. Of course if moving the sub helps, it could be the reduced vibrations from being further from the TV or the magnetic shielding issue.
Sorry for the trouble you are having, frustrating problem.
