[Grubhead]

I slightly disagree with Paul here. I don't assume a person has no experience of electronics from one short post. I bet there are a lot of trained engineers out there who would randomly replace an electronic part just on a hunch and probably get it right, by chance than training. However it seems to me you have some grasp of the equipment needed to service an amp so here's some general advice.
Fuses blow due to an overload. The most common fault being a shorted semi-conductor.
Acting like it was a piece of wire. An ohmmeter will show which part is shorted. So if you have one of them go hunting around for the short.