The head units don't have a pre-out. A lot just use the rear speaker feed and use a speaker to line converter. It's a bodge though as you don't have any real control of the sub and any speaker tone (bass) adjustments you need to make will affect the sub. IIRC all of the speaker outputs on the MMI are frequency filtered so it won't go as low as you'd like.
The best option on the basic sound system is to do the Audi Sound System retrofit, this enables the subwoofer speaker outputs on the head unit (and turns on the built in amp on the headunit). You can then use any sub with the appropriate ohm rating directly, or again use a line converter to feed a separate amp - if you use an auto sensing amp, you will only need to run speaker cables to the rear. When the headunit is modified you also get a separate subwoofer control on the Tone menu to set the output level.
In this scenario, you do need to fit the dash centre speaker to avoid an error being logged (only on diag tools, the car shows nothing) and you should change the front door bass speakers though I when I did mine I left the factory speakers whilst I got it up and running and they work fine (they have a different ohm rating on the Audi SS version allowing the amp to drive them harder).
