What was the setup with the Custom, any silencers? Cats/Decats? With a backbox, you're not going to get any louder than you did with the custom.
Mines custom, and when I saw it on the floor (I asked for a mid sport) (But they gave me a full sport setup) - It was a straight pipe from the Y pipe, no silencers straight to the back box. I was worrying all day that it was going to be too loud. Mine also has decats. But the backbox works miracles. It's quiet as you say on idle.
Cheapest option (May not be the best) Go back to custom exhaust, buy it back off them, and cut open the back box and strip it off all the baffles etc, should bring it to life.