It just dawned on me that the OP's going to swap out a fuse in his amp and my mentioning of the 2-3db difference was after replacing the 4 fuses in my SACD player. It was just out of boredom that I reversed them and got the results.
It was in the heat of this "discussion" that @georgehifi stated his doubt about a difference in direction making that much difference with a mains fuse. Just wanted to clear that up lest another can 'o worms opens when the OP reports back. When I changed the fuse in my integrated, it went in in the correct direction. The sound improved so there was no need to reverse it.
All the best,
Nonoise