wiring gauge and fuse combination?

Hello everyone. I'm new to the forum so I apologize if this has been answered before or I'm doing something wrong. I just have a couple quick questions and any help is much appreciated. I'm installing a sound system in my car and am using a Kenwood KAC6202 amplifier. I am using 8 gauge power wiring as that is what the manual for the amp says to use. What Amp fuse would be ideal for the 8 gauge wire from the battery? I already have an 80 AMP inline fuse holder from a previous install, but I don't know if that would work on this as well? Also, would a 4 gauge ground wire be okay if I'm using 8 gauge for my power wire? Thanks in advance.

Last first, yes, a 4 ga. ground wire is fine to use even when the power wire is smaller.  The amp manufacturer provided you with the minimum wire size you should use.  You can certainly upgrade that to a 4 gauge if you want to. 

A fluse holder with 80 amp fuse is fine to use with an 8 ga. wire.  If you were to use a 4 ga. power wire, you could fuse it at 150 amps.  https://www.the12volt.com/info/recwirsz.asp

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

Awesome. Thank you so much for your help.