Usually a ground hum caused by an amp/pre-amp interface will be constant in volume, i.e. not change with an increase in volume. The volume control, typically, only attentuates the signal from the source component into the pre-amp. The pre-amps output is typically constant, just as you amps out put is also. So what has changed with your sources, anything at all? Get a cheater plug(s) from you hardware store and start off by using it with each component and see if the hum goes away (its always handy to have one anyway.
Or, perhaps the most certain way is to work backwards. Disconnect all of your components, both IC's and PC's, and run only the amps into the speakers. Any hum? Plug in the pre-amp (IC's and PC's) and hum? No. Go to each source and repeat. You should find it that way. When you do put a cheater plug on the PC to the offending component.
DWIW.
Or, perhaps the most certain way is to work backwards. Disconnect all of your components, both IC's and PC's, and run only the amps into the speakers. Any hum? Plug in the pre-amp (IC's and PC's) and hum? No. Go to each source and repeat. You should find it that way. When you do put a cheater plug on the PC to the offending component.
DWIW.