im using a redback twin 2.5" cat back in mild steel. its the standard kit and does not drone at all. I have a 4" dump going through a standard 'hiflowed' cat. sounds quite deep at idle. I would prefer it to scream a bit more on wot, but im not willing to risk any droning as I do alot of highway driving- 110kph= 2100rpm= drone. you can get a cat back fitted for $700ish.