headers aren't.

why is this?

 

For one, the mufflers and CAI can be installed by a dealer and can keep the warranty on the powertrain.

 

Header will increase the loudness of some of the axlebacks out there. I know it did on my TRD exhaust. It made it a bit louder but it also deepend the tone a bit. Some exhaust have a tendency to sound very raspy when coupled to a header.