Usually, hum is caused by a ground loop. Verify that all audio equipment is connected to the same power source. If you are not using a power conditioner, we highly recommend that you add one. Not only will this help to minimize hum, it will better protect your equipment from power surges, brownouts, etc.
Use balanced cables whenever possible. If your audio device does not offer a balanced output, you can connect it to a DI (direct-injection) box, which will provide a ground-lift switch and a balanced output.
Finally, make sure that your audio cables are not run near power cables, and use cables that are the appropriate length for your application.
Using cables that are too long not only increases the risk of noise, it increases the likelihood that the cables are coiled, which will essentially create an antenna that picks up all kinds of audio interference.