Amp is a Sony 4 channel with 2 channels bridged for sub, the other 2 running 6x9's
Has blown fuse once but thats all.
Is this the stereo fuse in the fusebox?
do you never have it turned up all that loud?
A 4 channel amp should have at least 40A of fusing in total and HU fuses in the fusebox are normally only 20A. The wiring for the HU is only capable of about 20A down the full length of it ( like most internal car wiring). I am very surprised the fuse blowing is the only problem you have had, unless you never turn it up.
May not be fused!!
It must be fused unless someone has messed with the cars main fusebox. Or the HU wiring has been rerouted direct to the battery, but then I would have thought the amp power cable would also be there.