You don't need any of that. You can tap into the Bose amp to add a subwoofer. I did it on my B5.

Here is the pinout diagram on the bose amp plug.


You'll notice Audi used a common ground preamp signal. So when you splice in the RCA's to go to your new amp you have to do something a little different. You have to combine the two grounds on the wires into one and splice it into the common line level ground.

There is also the switched amp turn on signal available. All you have to do is run a power wire from the battery through the ECU box to the trunk.

 

You'll need keys to remove the head unit. EBAY

 

remove the stereo look at the back, if theres an area with two rca cable outputs all you have to do is connect this to youre amp with cables. you will need to bua a wiring kit because you will need to ground youre amp and run a power supply directly off youre batterie with an inline fuse. you will also need to connect a remote wire off your amp to tell it when to come on and turn off, this is usually a blue wire(the wires should be labeled). this black box they speak of is pertty much a ghetto way of doing it, what it does is steal the power from the stock speaker wiring, if you go this direction sound quality wont be as good, which will be pointless because you own an audi.

 

As far as I know there are no RCA preouts on the back of any Audi radio Sandman.

 

i just took my car to a local sound shop. i have a cap and amp for my subs and a 4 ch amp for my rear deck speakers (aftermarket)( bostons) and stock fronts. too much of a pain to do it my self. all this is still running off my stock delta deck.