ski hatch

dunno about the A6 but these are the normal two options in any saloon:

1) cut out section in middle of rear shelf if possible and trim with accoustic cloth/similar.
2) If you have decent amped speakers upfront in many people opinion the rears are pointless- the rear passengers can clearly hear the fronts. having rears only ruins the sounstage. So remove speakers if you have them on the rear shelf and the holes left will allow the sub to vent into cabin.

In addition to this try the subs facing different ways eg back front and straight up as will make a difference.

dunno if this will help but- Do you have a fold down armrest in the back? could always modify/remove the area behind that and pull down the armrest when driving so the sub literally shows from inside (unless using accoustic cloth to cover the area etc) and fires straight into the cabin- ive read about people doing this on the b5 iirc.