I just recently had a full audio system put in my 03 a6 consisting of two sound stream van gogh amplifiers, two sets of boston acoustic
sx-60 components, and one JL audio 15w3v2 in a sealed custom enclosure, i used my factory audi symphony2 with navi they wired everything up using hi-lo's and an alpine rca level bass ****, sounds awesome needs alot of dynamat!! next is the motorola bluetooth kit!

 

If you are keeping your stock Symphony II HU, you could always just add a line-out connector behind your Sym II to give you RCA outputs. See link for reference:

http://forums.bostonaudi.org/viewtopic.php?t=3945

Could be a much cleaner method.