Does anyone know what the offset was on the the 7.5" x 17" 9 spoke wheels that came stock on the 2004 A6 2.7T S-line

I do not, however my '02 A4 has 17x7.5's on it and they're +45mm offset. If you still have the rims look for the size specs cast into the inside ofthe rims. Mine (ET45) was right on the inside of one of the spokes.

Correct me if I'm wrong, but "S-line" is sport suspension?

Hope that helps.