I'm confused on this part. The signal going from the head unit to the amp is line level. If you tap into it there it would go into the line level (RCA) inputs on the new amp.
A line out converter takes a speaker output (high level) and converts it to line level (low level) so you can input it into the RCA jack inputs on an amp. It doesn't amplify the signal, it attenuates it.
William.