The 120 amp fuse is to protect the battery and wire mainly. If the wire got pinched off before your amp or was grounded some other way, the fuse would hopefully pop to prevent all your wire from being burnt up, starting a fire, and possibly discharging or wrecking your battery.
Using a fuse higher than 80 amps, but lower than your wire's rated capacity will not hurt anything.
Most people say fuse for your wire. Some people say fuse for your load. Still other people say fuse for both your wire and your load, using the lowest rating.
In this case, your amp has built in fuses. Therefore, it is protected from over current. I don't think you need to fuse for the load, just the wire. If your amp wasn't fused, that would be a different story.
Don't do what dub7Sonoma did with his fused amp.