you could also get a singe in dual out like i have. when i switched to the dual out from the singe out the tone got deeper and louder a little.
If you dont want to changer your exhaust, you have 2 options. you can either hollow out your cat which will result in a deeper louder tone. or you could change the size of your main pipe/tips. Generally the bigger the diameter main pipe and or tip will change the volume and tone of your exhaust.
heres the link to my exhaust post if u wanna take a look at what i did.
2002 CC, westin push bar with 2 lights, bed cover, custom made cat-back exhaust.......soon a steering stabilizer mod, lift, sounds system