How large of a gauge wire is in a fuse? How much wattage do you see going thru that?
Resistance is a measure of square area and distance. You could use 18awg wire if your jumper is only 6 inches long. It isn't going to matter.
Better yet, open up that crown amp real quick and look at the gauge wire used for the internal speaker wire. Bet it is 20awg or 18awg at most. Again, a short jumper does NOT have a lot of resistance, even at small gauges. It's the longer runs that matter.

By the way, are you using the 12awg on the 70V lines? You know that is super overkill right. One of the points of a 70V system, is the ability to use MUCH skinnier gauge wire. You can use 16awg for 200watts, up to 350 feet (according to some charts). Or 16awg for 400watts, up to 175 feet.