I had the Aeroturbine 4040XL, it sounded awesome, really deep and throaty, scared children when you fly by, but it was too loud and I drive around LA all day, it was also expensive, I think I paid $250 for it. I switched it to Rokktech Silent Might, and it sounds very similiar just not as loud, but still has the straight through design, so no restriction, so far I am happy with it, alot louder than stock and you can hear the Cummins growl. I still have the Aeroturbine in my shed, I would be willing to sell cheap if anyones interested.