I've just replaced and upgraded my original xenon bulbs in my 2009 facelift A6
The reason being, they were very bright, but didn't throw much light forward where you need it ! (If that makes sense)
Xenon bulb lose around 200 lumens per year on average
My car is equipped with D3S bulbs from factory.
I opted for a pair of Morimoto 5500k xenon bulbs from
@t8ups on here.
I also upgraded my cornering lights to Cree LED units to match the rest of the car (you have DRLs where these are on my car)
To gain access, on the O/S, remove the air box (complete), if you take the MAF and pipe to the turbo off in one it's simpler
This gets you plenty of space, the xenon bulb is right their, unclip the cable and twist to remove.
The N/S has alittle less space, but still do-able.
Unbolt the power steering reseviour - make sure you don't drop eithe the bolt (T30 IIRC or the spacer ring the is sandwiched in their too.
Open up the bracket, push and wiggle the reseviour DOWN until it's behind the intercooler
You know what access, all be it slights more fiddly due to the bracket remaining in place and the ABS pump.
You'll need really small hand to access the cornering light/DRL bulb (depending on spec) - these two bulbs alone took me 20 mins to replace as my xenons are directional (point left and right as well as up and down) so I have allot of motors and wiring tucked into the headlamp unit itself.
The end result, for me, ALLOT better !!
I have a positive throw of light in the centre, no light bleed and a very slightly whiter colouring, the overall performance of the headlights has been returned to standard if not better
Hope this helps
Tom