We have 2, 280MA amplifiers, both of which are creating the same result. Basically, I plug in an unbalanced source on any one of the 8 inputs and can hear a 60 cycle hum on both a 70V and a 4ohm speaker. If I balance the source using a DI, problem solved. I plugged the same unbalanced source into a Mackie amplifier and a Panasonic amplifier and was not able to replicate the sound. I am wondering if I have a possible warranty repair on my hands. Any input would be appreciated.