Invstbiker, if I were you I would start by disconnecting the preamp from the amp and see if the amp hums on its own.
300bs don't make a lot of power, so they have to be on efficient speakers. For this reason, they have to be hum-free. If, not, I would return it to the manufacturer for repair, unless there is a hum balanced control that simply needs adjustment.
OTOH if the system is quiet after this test, the next step is to disconnect all the inputs to the preamp and see if the preamp is quiet with amp and speakers.
Let us know what your progress is.
300bs don't make a lot of power, so they have to be on efficient speakers. For this reason, they have to be hum-free. If, not, I would return it to the manufacturer for repair, unless there is a hum balanced control that simply needs adjustment.
OTOH if the system is quiet after this test, the next step is to disconnect all the inputs to the preamp and see if the preamp is quiet with amp and speakers.
Let us know what your progress is.