well then you are going to input a high voltage signal into the amp and it will blow it. usually RCA input of the amp is from 0.5 to 8 volts (on high end amps) and 0.5 to 4 volts on normal amps. if you take the speaker out and feed it to amp directly, amp will get a high voltage then it can accept. keeping the volume of the head unit very low MIGT help if you can use the amp gain setting as volume control but this is not only impractical but also not advised. the filter thingy is for 150 rs i think.you can find it from any audio market, usually from the guy who sells wires and fuses etc.