strange problem with engine noise

the car is a 2001 civic sedan. sony cd h/u, with brand new kicker 4 channel amp (can't remember model #). i'm getting a static-like type of engine noise that increases with rpm. the 8 gauge ground wire is shorter than 18 inches and is solidly connected with scraping off of paint in the trunk, the power cable is connected straight to the battery with a 40 amp agu fuse. the rca cables go down the centre of the vehicle, and the pwr down the passenger side. the kicker amp has 2x20amp fuses on it. i've tried running a new set of good quality rca cables just free hanging in the air from h/u to amp to see if that will remove the problem, but nothing changed. i've tried another ground spot(s), running thicker pwr & ground wires, even new speaker wires, and still nothing changes. i also tried wiring thicker extra grounds at the battery to the chasis. so i figure it's the amp, and just for the sake of curiosity, i dig up an old alpine 4 channel amp that i had lying around with approximately the same pwr rating, and connected it to all the original wiring that was in place. son-of-a-gun.....no more noise! so i exchanged the kicker amp with a brand new kicker amp of the exact same model, and the noise is back to the way it was originally. now i'm just short of going crazy, but before i do, can you guys give me some suggestions? thanks in advance.

also if your head unit has 2 remote wires try swaping to the other remote wire(blue) and see what happens. one time i put a stereo in a toyota pickup truck and just because i was using one remote instead of the other the back speakers wouldnt work if the cd was playing but would work if i had the radio on. i swaped wires and it worked. if thats not the case try running a new power and ground for the head unit.yup

on most aftermarket h/u's, the blue remote output is active only when using radio. it's meant to be a power antenna turn-on. from what you've described, you probably used the h/u's speaker outputs to go directly to the front speakers and the rears were powered through an amplifier. oh, i forgot to mention that i've already used more direct sources of pwr and ground for the h/u. thank you for the reply though!