the good news-- you've only got a ground loop.
the bad news-- it's a ground loop.
in some cases, different amps are more sensitive to this then others, which might be the reason that your 4ch amp doesn't display any of the symptoms.
you were right to first check the ground, though you might also need to check the wire resistance between the ground and the amp-- it could be higher..
you can also try grounding the rca's, or grounding/powering the radio through the same wire the amps recieve power--sometimes the radio getting power from somewhere else can also cause these issues.
those are the main things i can think of at this time, though there are tons of guides and other suggestions all over the internet if the ones i listed don't work..