The internal fuse is to protect the amp in case the user is careless or doesn't know any better and replaces the blown fuse with a larger one. No need to worry about that fuse if the amp blows the circuit breaker. The fuse conducts and the tripping breaker proves it.
Easy way to find out if it's the transformer ( although most of the time it's NOT the transformer) is to disconnect the two red and one black secondary windings from the circuit, leave the primary windings hooked up to the AC power from the wall and try turning on the amp again. Make sure no unsoldered secondary windings are touching anything when you turn on the amp or sparks will fly.
If it does not trip the breaker the transformer is not shorted.
There could be any number of diodes or opamps or transistors shorted in the amp that are causing the fuse to blow. Most likely culprit is output transistors I would guess.
Fixing this amp will be hard for a beginner but it could happen. MOst shops would not work on it just for the fact that it is solid state and they don't want to waste time trying to figure out what the issue is, possibly not figuring it out and wasting an hour or two.