are you serious man? i got my slip on paired with hcf & xpipes and now i want to go catless bacause it's not loud enough, i actually think it's not even loud at all and there is zero drone to me even with the xpipes.
I have gone through quite different exhausts and it's the quitest one out of all of them, Gintani, AA, Borla and meistershaft, etc..
Check with akrapovic or your vendor to see if you've purchased the race version for the slip on.
