I'd be weary of trying this with a PC however with regular HU's generally the best way to eliminate ground loops is to ground the head unit with a large wire at the point where all the other audio components are located. IE: Ground distro block in the trunk. The reason being is RCA connections are unbalanced meaning they work by taking an AC voltage on the center conductor and comparing it to the ground of the sheild. Problem is when you have this grounded at multiple points some problems arise. First off the head unit is typically grounded closer to the source of power, which is the battery and the alternator. The amps which are some distance away are generally grounded at the rear of the car to the frame. Although it is very slight, the resistance of frame at the amps is generally higher than that at the point of radio grounding thus making comparing the ground reference at the source different than the amps, the amps see this as signal and thus create the ground noise.
Some things to try would be running a heavy gauge ground wire from the source to one of the ground terminals of the amps. Also try relocating the ground point of the amps to other locations making sure to sand down all paint at the point of contact. Cars are constructed of many pieces of steel and generally are only welded with a limited number of spot welds thus making the actual conducting area much smaller than you might think. Also be sure that all of your amps are grounded to ONE point, IE: if you have 2 amps and they just use a short cable screwed to the nearest point on the cars body. This can cause all kinds of fun interference in a cars electrical system.