KW V3 Variant 3 part number 35210064 Coilover for 2007-2011 S6 may also work with C6 Audi A6.  These were used for a few years on my S6 before I sold it about 25k miles on them. I paid $4000.00 for them in 2012.  Performed great and adjustable ride height produced an amazing look.  Buyer wanted the car stock.