The vast majority of modern amplifiers have internal fusing. This will protect the amp. Thus, the inline fuse on the wire needs only protect... the wire, and should be sized accordingly. This being said, n2 is correct that fusing a bit higher than max current draw (assuming your wire is oversized) is a bit safer method. The best answer, imo, is to size the fuse somewhere between the amp's max current draw, and the capacity of the wire. getting much more specific than this is pretty much splitting hairs that you wont likely need/appreciate unless you compete in a sanctioned event that bases classes on fuse size.
Ive had some wine this morning, so hopefully that made sense.