I would try without the amp and see if that helps. I have a friend that got a bad amp with his Leaf Ultimate. They sent him a new one. Also, Mohu support is pretty good if you wanted to give them a try.
Different devices measure signal strength differently, so you cannot do an apples-to-apples comparison between your TV and the Roamio. If you share your signal strengths, there are plenty of knowledgeable people he who could tell you whether what you are seeing is usable. I understand that anything that is about 40% should be usable on the Roamio, but I am no expert.
Roamio OTA with 2TB drive and 1 Mini