You should be very careful when using "cheater" plugs or removing the ground prong on a plug.
Equipment ground is there for your safety, removing it eliminates electricity having a safe path to ground.
Solving the source of your problem - a difference in potential with the different grounds in your system - is the best way to solve the problem.
Try using a single circuit. If that's not practical, try using a single circuit for your amplifiers and another single circuit for your source components.
If this doesn't work, you'll need to identify the source of the hum. Unplug a single component, one at a time, to see if it removes the hum - trying the preamp last.
Once you find the source(s), try plugging them into a circuit common with other parts of the system. Say a transport is the problem and it's plugged in a separate circuit, try plugging it in with the rest of your source components, or your amplifier(s) - in that order.
If all your equipment is plugged into a common circuit, than your equipment is suspect and it is NOT recommended to remove the ground prong of the plug.
Good luck and hope this helps.