Quality tape applied correctly can stay on without unraveling. For a absolutely quality splice for looks and durability, i would tape then heat shrink.
i would not crimp when i could solder signal wires.
you need the lower heat level on the gun and be quick on heating the wire, having held tight and steady in the air is best.
it takes maybe a few practice splices to get good enough with speaker wire, that is a very forgiving application. get some extra speaker wire and practice 3 or 4 splices, 20 minutes time.
after connecting both splices give each a wiggle, you should see no movement in the splice. disconnect wires from the amp and twist together with a wire nut (do this on the amp end only) and measure the resistance on the speaker end, it should be zero resistance.
add on the same gauge wire as in the wall.
yes a poor splice could make the wire equivalent to smaller or no wire.