Hey so I am new to all of this, and I have honestly been looking around on how to accomplish my goal and have found no good answers.

My question is as follows:
I have a 2006 A4 2.0T Quattro, Factory Stereo Deck, and I want to install a free-standing subwoofer in the trunk of my car. I have heard mixed things about whether you can do this with a factory head unit/deck. I don't want to have to lose my factory head unit because I have already put in Audi's XM sat radio, and there would be a huge ugly gap, lose my sat radio and my tape deck where the new head unit would go, so replacing the head unit isn't an option for me.

1) Is it possible to install subs with factory head unit?
2) If possible how do I install it?
3) What components do I need to install it?

Thanks a bunch!

signal inverter most stereo shop carry then for $30 taps in to the rear speaker leads and converts the signal in to a rca jack, mine had a remote turn on lead for the amp as well so the only wire i had to run was the power to the battery

FYI, there wouldn't necessarily be a huge ugly gap if your factory stereo gets fried. and you wouldn't have to lose your XM sat radio, either. chances are, there's an aftermarket head unit that will fit where your factory stereo currently is, and most decent aftermarket head units are XM/Sirius ready, and you could probably find something online that would show you how to install the head unit and still get your sat radio. hopefully this alleviates some of your worries about what happens if your stereo does get fried.

I am trying to do the same thing. I really like my stock factory unit. Ive heard that not getting an after-market unit will make the bass sound bad and not as "crisp"? Is that true?

I used the hi/lo level converter which just turns the wires into rca connectors which you hook right up to the amp. And as far as sound it sounds just as good to me so there isn't really any difference, so don't worry about the sound because it won't affect that. And for your power turn on you can hook that wire up to the CD changer in the back if you have one.