You'll never need 8GA but in this scenario (nor will that amp ever need it, ever- just a marketing gimmick to imply 'big power' )
Run 8GA with a good fuse from the battery- if you're gonna go thru the effort use the larger wire, you'll only need to run it once in case you upgrade later.
From there, put it into a distribution block
then go to whatever wire that amp uses.
Don't forget a good ground.
Done