300W amp turns on, no sound.

When routed the signal through the LINE OUT into a smaller guitar amp, I managed to hear the signal.
When routing the signal thru the 20W amp, it makes the signal come out quiet at high volumes.

I checked the fuses (4) they look good, except for one in which the inner rod has a little ball in the middle (due to hight heat / nearly burnt ? )
[the ball im talking about looks like when you make a little ball of soldering iron, but it doesnt look burnt or black at all]

I just need someone to awsner :
Could it be that a faulty fuse isnt letting the signal sound thru the original combo speaker, but does let the signal be routed thru the LINE OUT and fed into a different amp (as I stated above, being this the reason for the low signal at high master volume) ?

Any ideas of what is making the amp work in such a manner?

Im no expert in this, but I hope that with some trial and error, and problem discarding, I will manage to fix it.
Thanks for reading and reply with whatever your thoughts are, any help / comments / experience pointers are welcome...
:]

Maxx, December 2013