The #1 question is what you need to get. The majors except PBS are all "the same place", close, and STRONG. Even an extremely directional antenna wouldn't normally be able to discriminate among them. In your case, I think, IF THOSE SUFFICE, that you should eliminate the amplifier and splitter(s) and try with one TV. The amplifier may be causing some of your problem. With those signal strengths, if any amplifier is needed to overcome splitter and cable loss, it would be important for it to be one with excellent overload capability.
It has been 40+ years since I lived in Raleigh and I've forgotten where the FM towers are ... if one is close and/or strong, it also could be hurting your preamp.
Try one TV and get back to us.