Line Output Converter issues...
 

I have an 01 Monte Carlo SS with a factory amp under the rear trunk lid. I have the stock head unit. I have two 12 inch kickers and a Rockford fosgate amp. I have a line output converter spliced into the rear speaker wires going into the rear speakers. The subs I had before sounded bad and I thought it was the subs but I have brand new ones and still the sound quality is not so good. When I shake the line output converter wires that are spliced into the rear speakers the subs make random bass noises. I'm wondering if there is a better place to put the line out converter (before/after factory amp?) or if the converter is just bad. Any input would help. Thanks!