You have whats basically a "ground loop" this caused by the difference in electrical potential at different grounding points in your audio system. (All the grounds in a system should ideally be at "0" potential.) A ground loop typically adds a loud low-frequency hum or buzz as soon as you plug in any of various audio components, including Amp/amps, EQ's, H/U, Stackers etc. The buzz/hum is a byproduct of the multiple power supplys, power cables and a ground voltage differential within your system and its network of interconnecting cables.
A few checks...
Make sure the amplifiers have a good audio ground reference. In order for the amps to function properly, the audio ground must be referenced to chassis ground at the source unit. If it is not, the amplifier could oscillate. To check for a good ground reference, take a volt-ohm meter (VOM) and measure the resistance between the chassis of the radio and the shield of the RCA line level outputs of the radio. This reading should indicate a direct short. If this is not the case, grounding the shield of the RCA line level outputs to the chassis of the radio will probably be necessary. Keep amplifier power ground wires as short as possible. The longer a wire, the more resistance it has. When a current flows through a resistance, a voltage drop is produced. Because of this, the ground reference at the amplifier's circuit board is no longer the same as that at the chassis of the vehicle. This ground potential differential can lead to noise and improper operation of the amp. This is the same between amps and other stereo components. All should be nearly the same resistance back to the negitive terminal (yes I know - not easy to obtain).