Might be a ground loop issue.
Try disconnecting all of the inputs from the amps (after unplugging the amp power cords), but keep the speakers connected. Then, power the amps up and see if you get a buzz or hum. You could even try them one at a time, seeing as how they are monoblocks. If you get a buzz/hum without anything connected to the amps except for the speakers, then you know that it's not your inputs that are causing it. Be sure to double-check your speaker wire connections, too.
Please let us know the results of that.
Once you get it set up and don't have this issue, I'd appreciate reading your opinions of the Outlaw monoblocks. I've looked at those myself. Thanks.