I believe the feather bed would go under the sheet...We had one when I was a kid and that's what we did.

A feather bed goes under. A down comforter goes over

I think fiber bed is durable for long lasting whereas feather bed is slightly softer than fiber but it not much more durable comparison to fiber bed. If feather bed are wet then it may be infelicitous. But fiber bed is cleanable and its not affected by water.

Pillow Feather Bed which is one of the characters from Lalaloopsy is made from a baby's blanket.

The cast of The Feather Bed - 1933 includes: Jeanne De Casalis Gus McNaughton as Mr. Feather

The basic feather bed cover has 400 thread count sheets, which are very durable.