If the fuse is blowing off in an instant , it is sure that there is heavy current drain that makes this happen. So it will be ideal to proceed with an elimination surgery. You can disconnect the +B to the output sections one by one till the fuse does not fail. Now do not keep changing the fuse but have the amplifer connected through a limiter- you can use a 100 watts bulb in series to confirm till the bulb just glows ( when in high drain short the bulb will light up brighly)
Once the section is isolated then you can proceed to identify the component in the suspected circuit. Also make sure that the speakers are disconnected and the coils are not short as a failed output can cause damage to the speakers.
It is most possible that the output needs to be suspected.