Saab audio question
 

I just got an amp installed for a sub in my 2003 saab 9-5. I previously had this amp and sub installed in my for explorer sport trac to a custom radio and it hit hard and sounded great. I went to mickey shorr to get it installed in my saab (stock radio) and they hooked up the line output converter to the wires coming out of the head unit. I have a 10in polk audio sub in a ported box running off an alpine 500 watt monoblock amplifier. The sub is not hitting hard at all. I know it has something to do with the line out converter... Im not sure what though. I tried hooking up my ipod to the inputs on the amp and it hit very hard but when it is pluged into the line output converter coming out of the head unit it does not hit hard at all. And my amps gain is all the way up. Should the line output converter have been hooked up to the outputs of the stock speaker amp? any help is highly appriciated.