As a rule, amplifiers only help if the signal is strong enough at the antenna, but not strong enough to make it through the cables/switches/connectors/etc. to the normal receiver location.
What size dish were you getting the 76 and 51 readings on? I never even get the national HD channels that high on 61.5 on an 18" dish.
This morning, I now I have a signal of 3. Sometimes its really high, sometimes its really low. I just don't know.
As for the installer coming back, they never did, nor did they call. I called Dish explained this situation and they were disappointed that we've had all this trouble. They then set up an appt. for next weekend with a "field manger" who "won't leave until its working"......yeah right.