Cheap signal splitters can be tried as an attenuator. Any attenuator that you try should be put between the amp and the TV. Also, is there any possibility that the amp you got is sending a DC signal on the middle conductor? I'm assuming that the amp is one that plugs into the wall and not just an inline one that requires a DC voltage on the cable to operate.
"I'm just one story in a two story town." Bon Jovi