Whilst a Beolab 2 can produce a maximum of 850 watts of output power, that is NOT the power drawn from the mains socket, if it was you would have some very high electricity bills
Power drawn by a Beolab 2 from the 240 volt mains has a maximum of 15 watts running and 400 milli-watts in standby. A 3 amp fuse in my view is the correct fuse.
Also remember that power in a DC circuit is different from power in an AC circuit.