Nigle: I misread your post sorry. I see what you're saying. If I add a coupling cap to it I can block the dc but its going to take some work to figure out the size, otherwise I may end up blocking audio or on the other end allowing some of the dc to get to the amp. When I was explained about the project the term carrier was used. But you're right I'm confusing everyone.
Why would they do this ? Add in a 2.75V signal only to cut it out once its inside the old weak amp ? Noise immunity ? The whole thing ran over an rca cable, center pin signal, shield ground, I can't imagine they would do this for no reason, only thing I can think of is noise. Its always about noise