A quick search shows that amp is fused for a 60 amp peax (2 X 30 amp fuses) and that Sony recommends using an 80 amp fuse at the battery.
That kind of load would suggest 4 gauge, which is what Sony has designed the terminals to work with. That said, in my experience you could probably use 8 gauge with no issues since you'll seldom be pushing it at peak load.
A handy little site for working out wiring requirements can be found at:
http://www.powerstream.com/Wire_Size.htm