Where is the noise coming from when I hook several amps together and how do I fix it?

Please read everything!!! I have already tried many fixes. I appreciate any help, but I don’t want to be showered with things I have already tried!! Thanks!!

I have 3 Pyle Pro 3000 Watt Hybrid Amplifiers BIG PIC. They all have 2 inputs, 3 signal outputs, and 4 powered speaker outputs.

Inputs: Same stuff under different labels- DVD/CD and AUX

Outputs: Rec- Outputs original signal as it is recieved
PreOut- Outputs original signal leveled by main voume control
SubOut- Outputs only frequencies below 200 Hz with volume leveled by amps main volume control and separate SubVol nob

Whenever I try to rig them together lots of noise is created. For example, when I plug the PreOut to the AUX input of another. In this situation, noise is created in the amp that is OUTPUTTING the signal. Like just having it plugged in, not even playing anything. Changing the master volume of an amp DOES NOT change the volume of the noise, so even turning the volume to zero doesn’t get rid of it (WHY????????). Oh, and the noise fluctuates slightly. Its not a constant tone, it clicks occasionally and gets higher or lower.

Now for the observations!!!

I will describe a connection and then describe the noise. All the noise is generated in the amp OUTPUTTING the sound. So speakers connected to the amp outputting the signal to the other amp are affected. This noisy signal is then also sent to via the output to the other amp.

Rec to AUX- Least amount of noise, more high pitched. Stops if amp receiving signal is shut off.
PreOut to AUX- Lot more noise than Rec, stretches into lower frequencies. Stops if amp receiving signal is shut off.
Sub to AUX- Worst noise level. Same as PreOut, just louder. PERSISTS WHEN RECEIVING AMP IS SHUT OFF!

Ok, now some fixes I have tried and their results.

Thought: Their grounds aren’t synced up.
Action: Connect the grounds on the powered output channels.
Result: no change

Thought: Maybe the lights on the front are interfering???
Action: Dim the LED’s on the front panel.
Result: Noise quiets down and loses the lower frequencies. (WHY????)

Thought: I give up, I’ll try anything.
Action: Connect the signals on the powered output channels.
Result: Very weird. One amp shorts (don’t worry, built in protection), then noise changes. The noise persists, but its tone changes. (WHY???????)

Ok, just two questions: Why is this happening and how do I fix it?