loud hum from amp

I am trying to repair a CARLSBORO COBRA 90 BASS AMP with a very loud hum.seems someone plugged an extra speaker into it ,and did,not check the ohm,s.it kept blowing fuses all the time you switched it on.
I HAVE REPLACED THE FILTER CAPS,TRANSISTORS SOME DIODES AND POWER RESISTORS..ALAS NO SOUND FROM GUITAR ,JUST A MASSIVE LOUD HUM.
Any help would be great.

regards
Andomando.