Try putting the old splitter back in and see if HD works in the main home theater room. The powered splitter you have might be the wrong signal range, it seems that you need to be using a device that does 950-2150Mhz.
What he said.
Agreed. I've gone down the amplifier road before, but maybe I'll have more luck this time. I'm also hoping that they have something at Radio Shack similar to that one Attack linked earlier. I know I've gotten other amplifiers there in the past. I really don't have much time to work with here...