A:Answer This is very strange , We haven't had this problem with our Polk Magnifi Sound bar.
It could be the type connection you are using, HDMI is the best then optical, then the old type video yellow red white wires.
We are using the ARC (HDMI) connection which is best between a modern TV and a sound bar with no problems. Optical connection is also very good.
Have you tried updating the firmware of the sound bar?
Have you tried resetting the whole system and re sync the sound bar and sub?