I had the same problem, and ended up spending a bunch of time trying to solve it.
I determined that it was my amp that was causing a ground loop. I had to put in a discrete 20amp circuit with a dedicated ground. In my case, I have my grounds going to my basement where the pipes connect to the town water supply. Consequently, I had to sink a separate ground rod for the amp circuit to get rid of the problem.
I would also make sure that you are getting a good consistent supply of power. We had too many people being serviced by the same transformer, so at peak times I would see a huge power drop. Again, I had 3 separate 20 amp cicuits pulled for my source components, amp and plasma. I then added a PSAudio power supply. Needless to say, it made a huge difference.
Shortly after I did all this, the power company put in an extra transformer after too many people were running their A/C's, so the power drops went away. But, I'm still getting much better sound, and no scan lines on the plasma.
Needless to say, I learned a bunch of lessons on power.
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