the S6 makes good torque over a wide band (not better, but a lot, and can use it better initially), not as much as the other 2, but only so much can be put down, it is traction limited, hence traction control
with awd you can put more torque down, 4 vs 2 wheel, as in Veyron, GT-r, etc.
so it makes good torque, can put more of it down, and has a launch control that can control it well, by 30 mph is 0.6 sec ahead of the AMG
from reading some stuff from TorSen, if the car is balanced correctly, 30:70 is best to launch
and since the S6 is nose heavy, the front may get more power than 30% since it will have more traction due to weight
the 1/4 bears this out, within a few tenths in ET, but big difference in speed
so the Audi gets a much better launch
at 30 S6 1.2 vs 1.6 for the M5 and 1.8 for the AMG, it's 1/2 sec ahead
by 60 they are even
by 100 they've pulled a second on the S6
it all launch and awd, but after hooked up, that advantage goes away