Went to Dan Fast Muffler & Brake and got a 3 inch custom cat-back Magnaflow exhaust installed. I did get a resonator install just so it wasn't too loud. So far it is sounding great! I know over time the sound will change so we will wait and see what it sounds like several months later. I do have to say the S4 for sure feel much more torque'e... if that is a word. Most people say they feel more power at the top end with a exhaust but I feeling it more a the bottom end. Maybe it is helping the turbos spool up faster. It was around $550 for parts and labor so I'm happy with the end result!