I'm sorry that you're having trouble with your amp. Every amplifier goes through a full suite of bench testing in the lab before heading to the line for further testing and then packing. If there was a simple ground loop issue you would NOT only hear hum, there would be music as well.
The first step would be to disconnect all of the input (RCA) connections going to the amplifier and disconnect all of the speakers, except for your front left and right. Turn on the amp and see if there is any noise (there shouldn't be). If you DO hear the same hum, there is indeed an issue with the amplifier. If there isn't any noise, then turn off the amp and connect only the left and right front channel RCAs to the appropriate inputs on the amp for those channels. Power up again and see if you have noise. If you do then there is an issue with the outputs of your receiver, or the cables. Please get back to me with your findings.