The Bose sub runs off the main Bose amp. The problem with it is the flimsy plastic enclosure it resides in, the same one as the non Bose unit. You can tap into the loom to the main Bose amp in order to get the required feeds for a proper amp & sub.
All the info is here: http://www.dizzythedog.com/index.php?f=data_audi_a3_stealth_sub&a=1
As said the non Bose system also has a flimsy sub which also houses an amp for the sub and rear speakers.