As an aside, I've also noticed mechanical vibration causing distortion of the picture. This isn't caused by magnetism - its from physical vibration movement of the yoke. I see it when running my speakers, and they are physically too far away for magnetism to interfere with the yoke.

A trick we used to do with aircraft power supplies when x-formers
were in close proximity to sensitive circuitry, was to encase the
transformer in mu metal. The metal looked like the galvanized steel
used to construct old milk boxes (back in the 50s) and 1/32 of
an inch thickness would make short work of the surrounding flux.

I bet one sheet of this metal inside the speaker enclosure
facing the TV would do the trick. The only drawback would be cost.

Sincerely, Bill