I took a look at your AVR, you are going from RCA to either XLR or 1/4" in the sub amp. How are the cables or adapters constructed?
If you power on your sub amplifier with no input cable connected to it ( totally isolated from anything else ) does it still cause hum in the subwoofers? I read you had hum with only the amp connected, but was there an input cable connected? If so, try without the cable.
I am not advising you remove a safety ground, that is never a good idea. Having multiple devices with different ground potentials can cause noise issues.
Ask your doctor if DIY is right for you. Side effects of DIY may include anxiety, elevated blood pressure, lightheadedness, rapid heartbeat, skeletal muscle flaccidity, euphoria, psychological dependence, insomnia, confusion, blurred vision, impulsivity, uncontrolled or repeated movements.