I ended up taking the muffler off and replacing it with the resonator. It took out about 50% of the drone in the cab. I'm almost positive that if I could have installed it closer to the down pipe it would have worked way better.
I just ordered one from CP also. Going to start getting everything together for when the tuning comes out for my 15! Can't wait! I got the 30" stainless 4". Hope it's as quite in the cab as everyone says it is!