I have greddy ti-c exhaust and stillen gen 3 intake, i love the way it sounds but i just want it louder, i want to keep the "deeper" sound i have as much as possible. which of these 3 things would change the sound the most and make it louder, also at what point do i need to get it tuned?

 

HFC's or an aftermarket y-pipe

 

Agreed. If your just going for sound, header are going to cost you and arm and a leg.