Wetsounds Syn2 - 8 gauge wire adequate for power?

My boat has 8 gauge wire running to current amp. Wetsounds says 4 gauge for power for the syn series amps.

If I pull my amp and replace it, is the upgraded wire necessary - it will be a lot more work. Does it make a difference if only running one set of speakers (rev 8s) vs. 2 pair or larger speakers? I'm not very knowledgeable about electrical engineering, and don't understand DC vs. AC power requirements, but I've got large amplifiers with more wattage than these for my home stereo system that use standard powercords and I don't think those are larger than 8 gauge.

Also: my current wire is coming from a 30 amp fuse, but the syn2 has 3 x 25amp fuses on the back - I assume you really don't need 75 amps to run this one amplifier, but is a 30 amp fuse adequate?

trying to figure out how much I need to do to upgrade vs. leaving the stock in place. Appreciate any help.