Well, there's not much more you can do if you've still got the stock mid-pipe, which it sounds like you do. There's really no room to add any additional resonators.
We had that set-up with an off-road X pipe. It was extremely loud, but it droned like no other. If I were to do it again, I'd go with an off-road H or a catted X. However, that won't solve your issue. The only thing you can do is look into quieter mufflers to replace the ones in the Cervinis kit. I don't know how much luck you'll have, though, as that style of muffler is usually a "race" muffler and therefore usually not quiet.
Is it too loud, or is it just droning?