Well, if you've "already tried everything" I guess theres no hope.... Its time to give up TV and start reading books.....

Of course, if the following arent included in "Everything" then these might help....

Break the Ground on the Cable. Xantech, Tributaries, Mondial, and a few others make "Ground Breakers" that lift the cable ground. This is a common cause of Hum bars.

Surege/Filtering. Panamax, Monster Power and others offer Substantial Filtering of Cable TV lines in their higher end SUrge protectors. See if a Local dealer will let you audition one.

If its Every TV in the house, call the Cable company and have them Measure your level, Make sure the signal getting to the house is strong enough.

Bypass the AMplifier, and run direct to ONE TV. Do they go away? If so, maybe time for a new amp. The DA 500 has been disco'd for some time.....

Thats a start