Urgent amp help!

[Killeybh]

Everytime i reconnect the neg terminal on the battery the fuses in my amp go. It was 2 20 amp. i tried to 25 and they both went. Ive got a 600w amp but using a 1200w cable set as i got it off a mate for free and hes only just told me that they were 1200w. I thought all cables would be similar and the amp would jus take the power it needs as long as the cable can handle it. Is this right? could the fuse in the power cable be gone? i looked at it (its a weird tube thing) and it looked fine. how can i fix this?

thanks