Check the 10 ohm 2 watt resistors behind the RCAs they blow open when the amp get abused to prevent current from flowing back into the HU. I actually stock them just for this failure mode its pretty common. They may look perfectly Ok but you must ohm them out and they usually open up completely but I have seen some 1 k ohm readings before , so anything not 10 ohms replace them.
When they blow open the ground of the input is open so you can get hum and noise issues even though the amp works somewhat. Open gain pots can also cause issues similar to your description.