You can buy the Z06 mufflers used if you watch for them. They fit the base C6 with an M6 with no problems. There may be minor tweaks to get the Z06 cans to work on an A6 car. Add the mild to wild controller and you're set.
The NPP mufflers (dual mode option on the the base C6) are harder to find. You can buy them new from GM Partshouse or Scoggins Dickey (sp?) at prices typically less than local dealers, but shipping costs may wipe out the savings.
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