Hum from Plasma TV

to start with, some possibilities...

1. Is TV and hifi on the same AC line? If so, plug TV into a different AC outlet/breaker to prevent ground loop.
2. Is the speaker too close to the TT or does it not have enough isolation? Vibration from speaker can go thru the furniture, then cartridge and tonearm.
3. Are u using a grounding wire from the TT?

Try lifting the ground on 3 prong plugs by using a cheap 3-2 prong gray plug adapter from the hardware store. Try the TV first, then TT related equipment.

I did all the that stuff and more. What I'm thinking is the cables from the TT are picking up interference directly from the screen when the tv is on.

Maybe try some kind of shielded tone arm cable? I would move your TT as far away from the TV to see if anyhting happens. It's free and it might present a clue.