JBOO37

I just bought a KT-SW 15" Sub. As you know this is slightly older and I guess the type of amp is a different hook up than the new 6 and 7 channel receivers. Anyway I have an Onkyo TX-SR503 which only has the single RCA type pre out sub hook up(no regular speaker cable connections). The Sub has two sets of RCA hook up and several speaker wire posts, but does not have one for the single LFE. Do I need an adaptor? Help please! THX...