Why would a pre amp blow fuses?

Fuses blow most of the time due to an over current condition. You may have a cap or maybe a diode thats breaking down. Does the fuse blow upon powering up or after long times of being powered up.
UNDER NO circumstances replace the fuse with a higher value. If you do you will find out whats causing the problem most likely after causing considerable damage not to mention a fire hazard.

this all happened last month and I took it to a local shop they told me that my transformer was toast and the whole thing was junk.

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The transformer is a suspect. In THEORY if some of the windings started to short that would lower the resistance and raise the current level. Its possible that the humming maybe the windings vibrating - wearing away the insulation (enamel). Thats not always the case with a hum, its more of the exception.