i had my subs hooked up on my old hu (pioneer dehp3500) now i got a new car and im using just the stock hu until i get the clarion touch screen. anyways my amps gain has to be almost all the way up now on the stock hu compared to only about 1/4 of the way before on my pioneer. does this have anything to do with using the line converter?
The pioneer head unit probably had a higher voltage sub pre-out, a line converter works off the rear speakers, an amp can only work with what its given. (YES, YOUR INITIAL ASSUMPTION IS MOST LIKELY CORRECT)