fuse size

I've purchased a Sundown NS-1 and it doesn't have internal fuses and I'm scared of blowing it up. In the manual it says that the amp recommends a 650a fuse and they don't make 650a fuse also the amp has 3 sets of ground and power do all 3 power cables need to be fused off my second battery?

Ok forgive me if this is a stupid question... I've had to do a lot of research into car audio having not much experience in the past, but after buying my 2011 f250 lariat and being told about "premium sound" i was kind of pissed when i cranked it up and it sounded like a **** wind up radio... so...

I'm not sure what size fuse I need for my power wire. I believe it is a 4 gauge wire. I have a 50 amp fuse and its in a maxi fuse holder, but the slits in the holder are too loose and the 2 prongs in the fuse don't touch on each side. So i'm probably gonna get an anl fuse and holder instead...