Hello, I'm a 5th great grandaughter of Amasa Sr. via Amasa Jr.'s son Chester. You probably have all this by now, but thought I'd add here for you and anyone that may stumble on this thread.
For a while at least Amasa and several siblings were in Franklin County, New York, I am told he died and is buried there. I know his sons, Chester, Simon and Brewster were in Chateaugay, Franklin, NY 1840, they all appear on the same page of that US census. Brewster lived out his life there from what I have.
By 1860 Chester and his family are in Warren, Lake County, Illinois next door to his brother Simon. Here his son James, my great-grandfather enlisted in the civil war. After the war James, father Chester and most of the family went to Van Buren County, Michigan where Chester died living with his daughter Dell (Butterfield) and he is buried with her family at Lakeview Cemetery.
I believe I found Simon and wife Eunice in Monticello, Jones, Iowa 1870.