jaggerwild

Your problem is a common problem one in witch after the volume is turned up past a certain wattage the head unit will put out voltage to the RCA'S instead of a clean signal. This is more common on main stream heads such as Pioneer etc.... The volume goes to thirty we'll say for chits a giggles after passing the 20 or so mark they put out a lot of voltage and will generally kill a nice set up in no time.
One way to prevent voltage from reaching the Subs is to put a common light bulb from a tail light in-line with the source or positive signal. The bulb will act as a filter for the voltages and use it to light up removing the source but also will cut down the volume being introduced into the speaker. Thats the price you'll have to pay if you want quality not garbage coming from the speakers.

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JAGGERWILD