...related to the rear channel feeds are properly connected and isolated from feedback and interference, and everything is grounded properly, then it's probably a good bet that it's the amp itself. One thing you should check if it hasn't been done yet is to swap the front/rear lines to the amp, just to make sure that it's not the rear line-out in the h/u that's causing the problem.

In either case, if the gear's new and proven defective you should have them REPLACED and not repaired, just as long as you didn't sit on the stuff prior to install and allowed the return period to expire.